Journ. Geol. Soc.,' xxii. 185-218.

(9) 'Chemical and Geological Essays.' Sterry Hunt.

The above list only includes some of the more important memoirs which may be consulted as to the geological and chemical features of the Laurentian and Huronian Rocks, and as to the true nature of _Eozoon_. Those who are desirous of studying the later phases of the controversy with regard to _Eozoon_ must consult the papers of Carpenter, Carter, Dawson, King & Rowney, Hahn, and others, in the 'Quart. Journ. of the Geological Society,' the 'Proceedings of the Royal Irish Academy,' the 'Annals of Natural History,'

the 'Geological Magazine,' &c. Dr Carpenter's 'Introduction to the Study of the Foraminifera' should also be consulted.

[Footnote 10: In this and in all subsequently following bibliographical lists, not only is the selection of works and memoirs quoted necessarily extremely limited; but only such have, as a general rule, been chosen for mention as are easily accessible to students who are in the position of being able to refer to a good library. Exceptions, however, are occasionally made to this rule, in favour of memoirs or works of special historical interest. It is also unnecessary to add that it has not been thought requisite to insert in these lists the well-known handbooks of geological and palaeontological science; except in such instances as where they contain special information on special points.]

CHAPTER VIII.

THE CAMBRIAN PERIOD.

The traces of life in the Laurentian period, as we have seen, are but scanty; but the _Cambrian Rocks_--so called from their occurrence in North Wales and its borders ("Cambria ")--have yielded numerous remains of animals and some dubious plants.

The Cambrian deposits have thus a special interest as being the oldest rocks in which occur any number of well-preserved and unquestionable organisms. We have here the remains of the first _fauna_, or a.s.semblage of animals, of which we have at present knowledge. As regards their geographical distribution, the Cambrian Rocks have been recognised in many parts of the world, but there is some question as to the precise limits of the formation, and we may consider that their most typical area is in South Wales, where they have been carefully worked out, chiefly by Dr Henry Hicks. In this region, in the neighbourhood of the promontory of St David's, the Cambrian Rocks are largely developed, resting upon an ancient ridge of Pre-Cambrian (Laurentian?) strata, and overlaid by the lowest beds of the Lower Silurian. The subjoined sketch-section (fig. 27) exhibits in a general manner the succession of strata in this locality.

From this section it will be seen that the Cambrian Rocks in Wales are divided in the first place into a lower and an upper group. The _Lower Cambrian_ is const.i.tuted at the base by a great series of grits, sandstones, conglomerates, and slates, which are known as the "Longmynd group," from their vast development in the Longmynd Hills in Shropshire, and which attain in North Wales a thickness of 8000 feet or more. The Longmynd beds are succeeded by the so-called "Menevian group," a series of sandstones, flags, and grits, about 600 feet in thickness, and containing a considerable number of fossils. The _Upper Cambrian_ series consists in its lower portion of nearly 5000 feet of strata, princ.i.p.ally shaly and slaty, which are known as the "Lingula Flags," from the great abundance in them of a sh.e.l.l referable to the genus _Lingula_. These are followed by 1000 feet of dark shales and flaggy sandstones, which are known as the "Tremadoc slates," from their occurrence near Tremadoc in North Wales; and these in turn are surmounted, apparently quite conformably, by the bas.e.m.e.nt beds of the Lower Silurian.

[Ill.u.s.tration: Fig 27. GENERALIZED SECTION OF THE CAMBRIAN ROCKS IN WALES.]

The above may be regarded as giving a typical series of the Cambrian Rocks in a typical locality; but strata of Cambrian age are known in many other regions, of which it is only possible here to allude to a few of the most important. In Scandinavia occurs a well-developed series of Cambrian deposits, representing both the lower and upper parts of the formation. In Bohemia, the Upper Cambrian, in particular, is largely developed, and const.i.tutes the so-called "Primordial zone" of Barrande. Lastly, in North America, whilst the Lower Cambrian is only imperfectly developed, or is represented by the Huronian, the Upper Cambrian formation has a wide extension, containing fossils similar in character to the a.n.a.logous strata in Europe, and known as the "Potsdam Sandstone." The subjoined table shows the chief areas where Cambrian Rocks are developed, and their general equivalency:

TABULAR VIEW OF THE CAMBRIAN FORMATION.

_Britain._

_Europe._

_America._

/a. Tremadoc Slates.

a. Primordial zone

a. Potsdam

of Bohemia.

Sandstone.

b. Lingula Flags.

b. Paradoxides

b. Acadian Upper < schists,="" olenus="" group="" of="" new="" cambrian.="" schists,="" and="">

Dictyonema schists

of Sweden.

/a. Longmynd Beds.

a. Fucoidal

Huronian

Sandstone of Sweden

Formation?

b. Llanberis Slates.

b. _Eophyton_

Sandstone of Sweden.

Lower < c.="" harlech="" grits.="" cambrian.="" d.="" _oldhamia_="" slates="" of="" ireland.="" e.="" conglomerates="" and="" and="" sandstones="" of="" sutherlandshire?="" f.="" menevian="" beds.="">

Like all the older Palaeozoic deposits, the Cambrian Rocks, though by no means necessarily what would be called actually "metamorphic,"

have been highly cleaved, and otherwise altered from their original condition. Owing partly to their indurated state, and partly to their great antiquity, they are usually found in the heart of mountainous districts, which have undergone great disturbance, and have been subjected to an enormous amount of denudation. In some cases, as in the Longmynd Hills in Shropshire, they form low rounded elevations, largely covered by pasture, and with few or no elements of sublimity. In other cases, however, they rise into bold and rugged mountains, girded by precipitous cliffs.

Industrially, the Cambrian Rocks are of interest, if only for the reason that the celebrated Welsh slates of Llanberis are derived from highly-cleaved beds of this age. Taken as a whole, the Cambrian formation is essentially composed of arenaceous and muddy sediments, the latter being sometimes red, but more commonly nearly black in colour. It has often been supposed that the Cambrians are a deep-sea deposit, and that we may thus account for the few fossils contained in them; but the paucity of fossils is to a large extent imaginary, and some of the Lower Cambrian beds of the Longmynd Hills would appear to have been laid down in shallow water; as they exhibit rain-prints, sun-cracks, and ripple-marks--incontrovertible evidence of their having been a sh.o.r.e-deposit. The occurrence, of innumerable worm-tracks and burrows in many Cambrian strata is also a proof of shallow-water conditions; and the general absence of limestones, coupled with the coa.r.s.e mechanical nature of many of the sediments of the Lower Cambrian, maybe taken as pointing in the same direction.

The _life_ of the Cambrian, though not so rich as in the succeeding Silurian period, nevertheless consists of representatives of most of the great cla.s.ses of invertebrate animals. The coa.r.s.e sandy deposits of the formation, which abound more particularly towards its lower part, naturally are to a large extent barren of fossils; but the muddy sediments, when not too highly cleaved, and especially towards the summit of the group, are replete with organic remains. This is also the case, in many localities at any rate, with the finer beds of the Potsdam Sandstone in America.

Limestones are known to occur in only a few areas (chiefly in America), and this may account for the apparent total absence of corals. It is, however, interesting to note that, with this exception, almost all the other leading groups of Invertebrates are known to have come into existence during the Cambrian period.

Fig. 28.--Fragment of _Eophyton Linneanum_, a supposed land-plant.

Lower Cambrian, Sweden, of the natural size.

Of the land-surfaces of the Cambrian period we know nothing; and there is, therefore, nothing surprising in the fact that our acquaintance with the Cambrian vegetation is confined to some marine plants or sea-weeds, often of a very obscure and problematical nature. The "Fucoidal Sandstone" of Sweden, and the "Potsdam Sandstone" of North America, have both yielded numerous remains which have been regarded as markings left by sea-weeds or "Fucoids;" but these are highly enigmatical in their characters, and would, in many instances, seem to be rather referable to the tracks and burrows of marine worms. The first-mentioned of these formations has also yielded the curious, furrowed and striated stems which have been described as a kind of land-plant under the name of _Eopkyton_ (fig. 28). It cannot be said, however, that the vegetable origin of these singular bodies has been satisfactorily proved. Lastly, there are found in certain green and purple beds of Lower Cambrian age at Bray Head, Wicklow, Ireland, some very remarkable fossils, which are well known under the name of _Oldhamia_, but the true nature of which is very doubtful. The commonest form of _Oldhamia_ (fig. 29) consists of a thread-like stem or axis, from which spring at regular intervals bundles of short filamentous branches in a fan-like manner. In the locality where it occurs, the fronds of _Oldhamia_ are very abundant, and are spread over the surfaces of the strata in tangled layers. That it is organic is certain, and that it is a calcareous sea-weed is probable; but it may possibly belong to the sea-mosses (_Polyzoa_), or to the sea-firs (_Sertularians_).

Amongst the lower forms of animal life (_Protozoa_), we find the Sponges represented by the curious bodies, composed of netted fibres, to which the name of _Protospongia_ has been given (fig.

32, a); and the comparatively gigantic, conical, or cylindrical fossils termed _Archoeocyathus_ by Mr Billings are certainly referable either to the _Foraminifera_ or to the Sponges. The almost total absence of limestones in the formation may be regarded as a sufficient explanation of the fact that the _Foraminifera_ are not more largely and unequivocally represented; though the existence of greensands in the Cambrian beds of Wisconsin and Tennessee may be taken as an indication that this cla.s.s of animals was by no means wholly wanting. The same fact may explain the total absence of corals, so far as at present known.

[Ill.u.s.tration: Fig. 29.--A portion of _Oldhamia antiqua_, Lower Cambrian, Wicklow, Ireland, of the natural size. (After Salter.)]

The group of the _Echinodermata_ (Sea-lilies, Sea-urchins, and their allies) is represented by a few forms, which are princ.i.p.ally of interest as being the earliest-known examples of the cla.s.s.

It is also worthy of note that these precursors of a group which subsequently attains such geological importance, are referable to no less than three distinct _orders_--the Crinoids or Sea-lilies, represented by a species of _Dendrocrinus_; the Cystideans by _Protocyst.i.tes_; and the Star-fishes by _Palasterina_ and some other forms. Only the last of these groups, however, appears to occur in the Lower Cambrian.

[Ill.u.s.tration: Fig. 30.--Annelide-burrows (_Scolithus linearus_) from the Potsdam Sandstone of Canada, of the natural size. (After Billings.)]

The Ringed-worms (_Annelida_), if rightly credited with all the remains usually referred to them, appear to have swarmed in the Cambrian seas. Being soft-bodied, we do not find the actual worms themselves in the fossil condition, but we have, nevertheless, abundant traces of their existence. In some cases we find vertical burrows of greater or less depth, often expanded towards their apertures, in which the worm must have actually lived (fig. 30), as various species do at the present day. In these cases, the tube must have been rendered more or less permanent by receiving a coating of mucus, or perhaps a genuine membranous secretion, from the body of the animal; and it may be found quite empty, or occupied by a cast of sand or mud. Of this nature are the burrows which have been described under the names of _Scolithus_ and _Scolecoderma_, and probably the _Histioderma_ of the Lower Cambrian of Ireland. In other cases, as in _Arenicolites_ (fig.

32, b), the worm seems to have inhabited a double burrow, shaped like the letter U, and having two openings placed close together on the surface of the stratum. Thousands of these twin-burrows occur in some of the strata of the Longmynd, and it is supposed that the worm used one opening to the burrow as an aperture of entrance, and the other as one of exit. In other cases, again, we find simply the meandering trails caused by the worm dragging its body over the surface of the mud. Markings of this kind are commoner in the Silurian Rocks, and it is generally more or less doubtful whether they may not have been caused by other marine animals, such as sh.e.l.lfish, whilst some of them have certainly nothing whatever to do with the worms. Lastly, the Cambrian beds often show twining cylindrical bodies, commonly more or less matted together, and not confined to the surfaces of the strata, but pa.s.sing through them. These have often been regarded as the remains of sea-weeds, but it is more probable that they represent casts of the underground burrows of worms of similar habits to the common lob-worm (_Arenicola_) of the present day.

The _Articulate_ animals are numerously represented in the Cambrian deposits, but exclusively by the cla.s.s of _Crustaceans_. Some of these are little double-sh.e.l.led creatures, resembling our living water-fleas (_Ostracoda_). A few are larger forms, and belong to the same group as the existing brine-shrimps and fairy-shrimps (_Phyllopoda_). One of the most characteristic of these is the _Hymenocaris vermicauda_ of the Lingula Flags (fig.

32, d). By far the larger number of the Cambrian _Crustacea_ belong, however, to the remarkable and wholly extinct group of the _Trilobites_. These extraordinary animals must have literally swarmed in the seas of the later portion of this and the whole of the succeeding period; and they survived in greatly diminished numbers till the earlier portion of the Carboniferous period.

They died out, however, wholly before the close of the Palaeozoic epoch, and we have no Crustaceans at the present day which can be considered as their direct representatives. They have, however, relationships of a more or less intimate character with the existing groups of the Phyllopods, the King-crabs (_Limulus_), and the Isopods ("Slaters," Wood-lice, &c.) Indeed, one member of the last-mentioned order, namely, the _Serolis_ of the coasts of Patagonia, has been regarded as the nearest living ally of the Trilobites. Be this as it may, the Trilobites possessed a skeleton which, though capable of undergoing almost endless variations, was wonderfully constant in its pattern of structure, and we may briefly describe here the chief features of this.

[Ill.u.s.tration: Fig. 31.--Cambrian Trilobites: a, _Paradoxides Bohemicus_, reduced in size; b, _Ellipsocephalus Hoffi_; c, _Sao hirsuta_; d, _Conocorypke Sultzeri_ (all the above, together with fig. g, are from the Upper Cambrian or "Primordial Zone" of Bohemia); e, Head-shield of _Dikellocephalus Celticus_, from the Lingula Flags of Wales; f, Head-shield of _Conocoryphe Matthewi_, from the Upper Cambrian (Acadian Group) of New Brunswick; g, _Agnostus rex_, Bohemia; h, Tail-shield of _Dikellocephalus Minnesotensis_, from the Upper Cambrian (Potsdam Sandstone) of Minnesota. (After Barrande, Dawson, Salter, and Dale Owen.)]

The upper surface of the body of a Trilobite was defended by a strong sh.e.l.l or "crust," partly h.o.r.n.y and partly calcareous in its composition. This sh.e.l.l (fig. 31) generally exhibits a very distinct "trilobation" or division into three longitudinal lobes, one central and two lateral. It also exhibits a more important and more fundamental division into three transverse portions, which are so loosely connected with one another as very commonly to be found separate. The first and most anterior of these divisions is a shield or buckler which covers the head; the second or middle portion is composed of movable rings covering the trunk ("thorax "); and the third is a shield which covers the tailor "abdomen." The head-shield (fig. 31, e) is generally more or less semicircular in shape; and its central portion, covering the stomach of the animal, is usually strongly elevated, and generally marked by lateral furrows. A little on each side of the head are placed the eyes, which are generally crescentic in shape, and resemble the eyes of insects and many existing Crustaceans in being "compound," or made up of numerous simple eyes aggregated together.

So excellent is the state of preservation of many specimens of Trilobites, that the numerous individual lenses of the eyes have been uninjured, and as many as four hundred have been counted in each eye of some forms. The eyes may be supported upon prominences, but they are never carried on movable stalks (as they are in the existing lobsters and crabs); and in some of the Cambrian Trilobites, such as the little _Agnosti_ (fig. 31 g), the animal was blind. The lateral portions of the head-shield are usually separated from the central portion by a peculiar line of division (the so-called "facial suture") on each side; but this is also wanting in some of the Cambrian species. The backward angles of the head-shield, also, are often prolonged into spines, which sometimes reach a great length. Following the head-shield behind, we have a portion of the body which is composed of movable segments or "body-rings," and which is technically called the "thorax," Ordinarily, this region is strongly trilobed, and each ring consists of a central convex portion, and of two flatter side-lobes. The number of body-rings in the thorax is very variable (from two to twenty-six), but is fixed for the adult forms of each group of the Trilobites. The young forms have much fewer rings than the full-grown ones; and it is curious to find that the Cambrian Trilobites very commonly have either a great many rings (as in _Paradoxides_, fig. 31, a), or else very few (as in _Agnostus_, fig. 31, g). In some instances, the body-rings do not seem to have been so constructed as to allow of much movement, but in other cases this region of the body is so flexible that the animal possessed the power of rolling itself up completely, like a hedgehog; and many individuals have been permanently preserved as fossils in this defensive condition. Finally, the body of the Trilobite was completed by a tail-shield (technically termed the "pygidium"), which varies much in size and form, and is composed of a greater or less number of rings, similar to those which form the thorax, but immovably amalgamated with one another (fig. 31, h).

The under surface of the body in the Trilobites appears to have been more or less entirely dest.i.tute of hard structures, with the exception of a well-developed upper lip, in the form of a plate attached to the inferior side of the head-shield in front. There is no reason to doubt that the animal possessed legs; but these structures seem to have resembled those of many living Crustaceans in being quite soft and membranous. This, at any rate, seems to have been generally the case; though structures which have been regarded as legs have been detected on the under surface of one of the larger species of Trilobites. There is also, at present, no direct evidence that the Trilobites possessed the two pairs of jointed feelers ("antennae") which are so characteristic of recent Crustaceans.

The Trilobites vary much in size, and the Cambrian formation presents examples of both the largest and the smallest members of the order. Some of the young forms may be little bigger than a millet-seed, and some adult examples of the smaller species (such as _Agnostus_) may be only a few lines in length; whilst such giants of the order as _Paradoxides_ and _Asaphus_ may reach a length of from one to two feet. Judging from what we actually know as to the structure of the Trilobites, and also from a.n.a.logous recent forms, it would seem that these ancient Crustaceans were mud-haunting creatures, denizens of shallow seas, and affecting the soft silt of the bottom rather than the clear water above.

Whenever muddy sediments are found in the Cambrian and Silurian formations, there we are tolerably sure to find Trilobites, though they are by no means absolutely wanting in limestones. They appear to have crawled out upon the sea-bottom, or burrowed in the yielding mud, with the soft under surface directed downwards; and it is probable that they really derived their nutriment from the organic matter contained in the ooze amongst which they lived. The vital organs seem to have occupied the central lobe of the skeleton, by which they were protected; and a series of delicate leaf-like paddles, which probably served as respiratory organs, would appear to have been carried on the under surface of the thorax. That they had their enemies may be regarded as certain; but we have no evidence that they were furnished with any offensive weapons, or, indeed, with any means of defence beyond their hard crust, and the power, possessed by so many of them, of rolling themselves into a ball. An additional proof of the fact that they for the most part crawled along the sea-bottom is found in the occurrence of tracks and markings of various kinds, which can hardly be ascribed to any other creatures with any show of probability.

That this is the true nature of some of the markings in question cannot be doubted at all; and in other cases no explanation so probable has yet been suggested. If, however, the tracks which have been described from the Potsdam Sandstone of North America under the name of _Protichnites_ are really due to the peregrinations of some Trilobite, they must have been produced by one of the largest examples of the order.

As already said, the Cambrian Rocks are very rich in the remains of Trilobites. In the lowest beds of the series (Longmynd Rocks), representatives of some half-dozen genera have now been detected, including the dwarf _Agnostus_ and the giant _Paradoxides_. In the higher beds, the number both of genera and species is largely increased; and from the great comparative abundance of individuals, the Trilobites have every right to be considered as the most characteristic fossils of the Cambrian period,--the more so as the Cambrian species belong to peculiar types, which, for the most part, died out before the commencement of the Silurian epoch.

All the remaining Cambrian fossils which demand any notice here are members of one or other division of the great cla.s.s of the _Mollusca_, or "Sh.e.l.l-fish" properly so called. In the Lower Cambrian Rocks the Lamp-sh.e.l.ls (_Brachiopoda_) are the princ.i.p.al or sole representatives of the cla.s.s, and appear chiefly in three interesting and important types--namely, _Lingulella, Discina,_ and _Obolella_. Of these the last (fig. 32, i) is highly characteristic of these ancient deposits; whilst _Discina_ is one of those remarkable persistent types which, commencing at this early period, has continued to be represented by varying forms through all the intervening geological formations up to the present day. _Lingulella_ (fig. 32, c), again, is closely allied to the existing "Goose-bill" Lamp-sh.e.l.l (_Lingula anatina_), and thus presents us with another example of an extremely long-lived type. The _Lingulelloe_ and their successors; the _Linguloe_, are singular in possessing a sh.e.l.l which is of a h.o.r.n.y texture, and contains but a small proportion of calcareous matter. In the Upper Cambrian Rocks, the _Lingulelloe_ become much more abundant, the broad satchel-shaped species known as _L. Davisii_ (fig. 32, e) being so abundant that one of the great divisions of the Cambrian is termed the "Lingula Flags." Here, also, we meet for the first time with examples of the genus Orthis (fig. 32, f, k, l) a characteristic Palaeozoic type of the Brachiopods, which is destined to undergo a vast extension in later ages.

[Ill.u.s.tration: Fig 32.--Cambrian Fossils: a, _Protospongia fenestrata_, Menevian Group; b, _Arenicolites didymus_, Longmynd Group; c, _Lingulella ferruginea_, Longmynd and Menevian, enlarged; d, _Hymenocaris vermicauda_, Lingula Flags; e, _Lingulella Davisii_, Lingula Flags; f, _Orthis lenticularis_, Lingula Flags; g, _Theca Davidii_, Tremadoc Slates; h, _Modiolopsis Solvensis_, Tremadoc Slates; i, _Obolela sagittalis_, interior of valve, Menevian; j, Exterior of the same; k, _Orthis Hicksii_, Menevian; l, Cast of the same; m, _Olenus micrurus_, Lingula Flags. (Alter Salter, Hicks, and Davidson.)]

Of the higher groups of the _Mollusca_ the record is as yet but scanty. In the Lower Cambrian, we have but the thin, fragile, dagger-shaped sh.e.l.ls of the free-swimming oceanic Molluscs or "Winged-snails" (_Pteropoda_), of which the most characteristic is the genus _Theca_ (fig. 32, g). In the Upper Cambrian, in addition to these, we have a few Univalves (_Gasteropoda_), and, thanks to the researches of Dr Hicks, quite a small a.s.semblage of Bivalves (_Lamellibranchiata_), though these are mostly of no great dimensions (fig. 32, h). Of the chambered _Cephalopoda_ (Cuttle-fishes and their allies), we have but few traces; and these wholly confined to the higher beds of the formation. We meet, however, with examples of the wonderful genus _Orthoceras_, with its straight, part.i.tioned sh.e.l.l, which we shall find in an immense variety of forms in the Silurian rocks. Lastly, it is worthy of note that the lowest of all the groups of the _Mollusca_--namely, that of the Sea-mats, Sea-mosses, and Lace-corals (_Polyzoa_)--is only doubtfully known to have any representatives in the Cambrian, though undergoing a large and varied development in the Silurian deposits.

[Ill.u.s.tration: Fig. 33.--Fragment of _Dictyonema sociale_, considerably enlarged, showing the h.o.r.n.y branches, with their connecting cross-bars, and with a row of cells on each side.

(Original.)]

An exception, however, may with much probability be made to this statement in favour of the singular genus _Dictyonema_ (fig.

33), which is highly characteristic of the highest Cambrian beds (Tremadoc Slates). This curious fossil occurs in the form of fan-like or funnel-shaped expansions, composed of slightly-diverging h.o.r.n.y branches, which are united in a net-like manner by numerous delicate cross-bars, and exhibit a row of little cups or cells, in which the animals were contained, on each side. _Dictyonema_ has generally been referred to the _Graptolites_; but it has a much greater affinity with the plant-like Sea-firs (_Sertularians_) or the Sea-mosses (_Polyzoa_), and the balance of evidence is perhaps in favour of placing it with the latter.

LITERATURE.

The following are the more important and accessible works and memoirs which may be consulted in studying the stratigraphical and palaeontological relations of the Cambrian Rocks:--

(1) 'Siluria.' Sir Roderick Murchison. 5th ed., pp. 21-46.

(2) 'Synopsis of the Cla.s.sification of the British Palaeozoic Rocks.'

Sedgwick. Introduction to the 3d Fasciculus of the 'Descriptions of British Palaeozoic Fossils in the Woodwardian Museum,'

by F. M'Coy, pp. i-xcviii, 1855.

(3) 'Catalogue of the Cambrian and Silurian Fossils in the Geological Museum of the University of Cambridge.' Salter. With a Preface by Prof. Sedgwick. 1873.

(4) 'Thesaurus Siluricus.' Bigsby. 1868.

(5) "History of the Names Cambrian and Silurian." Sterry Hunt.--'Geological Magazine.' 1873.

(6) 'Systeme Silurien du Centre de la Boheme.' Barrande. Vol. I.

(7) 'Report of Progress of the Geological Survey of Canada, from its Commencement to 1863,' pp. 87-109.

(8) 'Acadian Geology.' Dawson. Pp. 641-657.