CHAPTER XXIX.

VOLCANIC ROCKS--_continued_.

Trap dike--sometimes project--sometimes leave fissures vacant by decomposition--Branches and veins of trap--Dikes more crystalline in the centre--Foreign fragments of rock imbedded--Strata altered at or near the contact--Obliteration of organic remains--Conversion of chalk into marble--and of coal into c.o.ke--Inequality in the modifying influence of dikes--Trap interposed between strata--Columnar and globular structure--Relation of trappean rocks to the products of active volcanos--Submarine lava and ejected matter correspond generally to ancient trap--Structure and physical features of Palma and some other extinct volcanos 378

CHAPTER x.x.x.

ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS.

Tests of relative age of volcanic rocks--Test by superposition and intrusion--Dike of Quarrington Hill, Durham--Test by alteration of rocks in contact--Test by organic remains--Test of age by mineral character--Test by included fragments--Volcanic rocks of the Post-Pliocene period--Basalt of Bay of Trezza in Sicily--Post-Pliocene volcanic rocks near Naples--Dikes of Somma--Igneous formations of the Newer Pliocene period--Val di Noto in Sicily 397

CHAPTER x.x.xI.

ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS--_continued_.

Volcanic rocks of the Older Pliocene period--Tuscany--Rome--Volcanic region of Olot in Catalonia--Cones and lava-currents--Ravines and ancient gravel-beds--Jets of air called Bufadors--Age of the Catalonian volcanos--Miocene period--Brown-coal of the Eifel and contemporaneous trachytic breccias--Age of the brown-coal--Peculiar characters of the volcanos of the upper and lower Eifel--Lake craters--Tra.s.s--Hungarian volcanos 408

CHAPTER x.x.xII.

ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS--_continued_.

Volcanic rocks of the Pliocene and Miocene periods continued--Auvergne--Mont Dor--Breccias and alluviums of Mont Perrier, with bones of quadrupeds--River dammed up by lava-current--Range of minor cones from Auvergne to the Vivarais--Monts Dome--Puy de Come--Puy de Pariou--Cones not denuded by general flood--Velay--Bones of quadrupeds buried in scoriae--Cantal--Eocene volcanic rocks--Tuffs near Clermont--Hill of Gergovia--Trap of Cretaceous period--Oolitic period--New Red Sandstone period--Carboniferous period--Old Red Sandstone period--"Rock and Spindle" near St. Andrews--Silurian period--Cambrian volcanic rocks 422

CHAPTER x.x.xIII.

PLUTONIC ROCKS--GRANITE.

General aspect of granite--Decomposing into spherical ma.s.ses--Rude columnar structure--a.n.a.logy and difference of volcanic and plutonic formations--Minerals in granite, and their arrangement--Graphic and porphyritic granite--Mutual penetration of crystals of quartz and felspar--Occasional minerals--Syenite--Syenitic, talcose, and schorly granites--Eurite--Pa.s.sage of granite into trap--Examples near Christiania and in Aberdeenshire--a.n.a.logy in composition of trachyte and granite--Granite veins in Glen Tilt, Cornwall, the Valorsine, and other countries--Different composition of veins from main body of granite--Metalliferous veins in strata near their junction with granite--Apparent isolation of nodules of granite--Quartz veins--Whether plutonic rocks are ever overlying--Their exposure at the surface due to denudation 436

CHAPTER x.x.xIV.

ON THE DIFFERENT AGES OF THE PLUTONIC ROCKS.

Difficulty in ascertaining the precise age of a plutonic rock--Test of age by relative position--Test by intrusion and alteration--Test by mineral composition--Test by included fragments--Recent and Pliocene plutonic rocks, why invisible--Tertiary plutonic rocks in the Andes--Granite altering Cretaceous rocks--Granite altering Lias in the Alps and in Skye--Granite of Dartmoor altering Carboniferous strata--Granite of the Old Red Sandstone period--Syenite altering Silurian strata in Norway--Blending of the same with gneiss--Most ancient plutonic rocks--Granite protruded in a solid form--On the probable age of the granites of Arran, in Scotland 449

CHAPTER x.x.xV.

METAMORPHIC ROCKS.

General character of metamorphic rocks--Gneiss--Hornblende-schist --Mica-schist--Clay-slate--Quartzite--Chlorite-schist--Metamorphic limestone--Alphabetical list and explanation of other rocks of this family--Origin of the metamorphic strata--Their stratification is real and distinct from cleavage--Joints and slaty cleavage--Supposed causes of these structures--how far connected with crystalline action 463

CHAPTER x.x.xVI.

METAMORPHIC ROCKS--_continued_.

Strata near some intrusive ma.s.ses of granite converted into rocks identical with different members of the metamorphic series--Arguments hence derived as to the nature of plutonic action--Time may enable this action to pervade denser ma.s.ses--From what kinds of sedimentary rock each variety of the metamorphic cla.s.s may be derived--Certain objections to the metamorphic theory considered--Lamination of trachyte and obsidian due to motion--Whether some kinds of gneiss have become schistose by a similar action 473

CHAPTER x.x.xVII.

ON THE DIFFERENT AGES OF THE METAMORPHIC ROCKS.

Age of each set of metamorphic strata twofold--Test of age by fossils and mineral character not available--Test by superposition ambiguous--Conversion of dense ma.s.ses of fossiliferous strata into metamorphic rocks--Limestone and shale of Carrara--Metamorphic strata of modern periods in the Alps of Switzerland and Savoy--Why the visible crystalline strata are none of them very modern--Order of succession in metamorphic rocks--Uniformity of mineral character--Why the metamorphic strata are less calcareous than the fossiliferous 481

CHAPTER x.x.xVIII.

MINERAL VEINS.

Werner's doctrine that mineral veins were fissures filled from above--Veins of segregation--Ordinary metalliferous veins or lodes--Their frequent coincidence with faults--Proofs that they originated in fissures in solid rock--Veins shifting other veins--Polishing of their walls--Sh.e.l.ls and pebbles in lodes--Evidence of the successive enlargement and re-opening of veins--Fournet's observations in Auvergne--Dimensions of veins--Why some alternately swell out and contract--Filling of lodes by sublimation from below--Chemical and electrical action--Relative age of the precious metals--Copper and lead veins in Ireland older than Cornish tin--Lead vein in lias, Glamorganshire--Gold in Russia--Connection of hot springs and mineral veins--Concluding remarks 488

_Dates of the successive Editions of the "Principles" and "Elements" (or Manual) of Geology, by the Author._

Principles, 1st vol. in octavo, published in - - - Jan. 1830.

----, 2d vol. do. - - - - - - - - - - - - - - - - Jan. 1832.

----, 1st vol. 2d edition in octavo - - - - - - - 1832.

----, 2d vol. 2d edition do. - - - - - - - - - - Jan. 1833.

----, 3d vol. 1st edition do. - - - - - - - - - - May 1833.

----, New edition (called the 3d) of the whole work in 4 vols.

12mo. - - - - - - - - - - - - - - - - - - - May 1834.

----, 4th edition, 4 vols. 12mo. - - - - - - - - - June 1835.

----, 5th edition, do. do. - - - - - - - - - - - - Mar. 1837.

Elements, 1st edition in one vol. - - - - - - - - July 1838.

Principles, 6th edition, 3 vols. 12mo. - - - - - - June 1840.

Elements, 2d edition in 2 vols. 12mo. - - - - - - July 1841.

Principles, 7th edition in one vol. 8vo. - - - - - Feb. 1847.