{111}

Put in a few fresh-water clams and insects in variety, water boatmen, diving spiders, and whirligigs. A tank of beetles will be full of interest. Always add two or three tadpoles as scavengers, and watch their legs grow out as the tail grows short and they become frogs. You can find or buy a variety of turtles which will soon be tame and eat from your fingers. Do not keep turtles with fish.

On every hike or tramp carry a wide-mouthed bottle for specimens and a piece of rubber cloth in which to bring home water plants. Fish can be carried wrapped in damp moss for hours and will be found well and lively when put in the aquarium.

Fish Food

Fish require very little food other than the minute creatures that develop in the water.

The dealers supply proper foods for aquaria, or you can prepare your own. Fine vermicelli is good for gold fish, sc.r.a.ped lean beef is just what the sunfish and Paradise fish want. Ant eggs suit many fish, and powdered dog biscuit will fill many mouths. It is evident that an article so brief as this is only suggestive. The libraries contain many books of which two are recommended:

"Home Aquarium and How to Care For It." By Eugene Smith, 1902.

Published by Dutton, New York.

"Book of Aquaria." By Bateman and Bennett, 1890. Published by L.

Upcott Gill, 170 Strand, W. C., London.

ROCKS AND PEBBLES

_United States Geological Survey_

Geologists study the materials of the earth's crust, the processes continually changing its surface, and the forms and structures thus produced. In a day's tramp one may see much under each of these heads.

The earth's crust is made up chiefly of the hard rocks, which outcrop in many places, but are largely covered by thin, loose, surface materials. Rocks may be igneous, which have cooled from a melted condition; or sedimentary, which are made of layers spread one upon another by water currents or waves, or by winds.

Igneous rocks, while still molten, have been forced into other rocks from below, or poured out on the surface from volcanoes. They are chiefly made of crystals of various minerals, such as quartz, felspar, mica, and pyrite. Granite often contains large crystals of felspar or mica. Some igneous rocks, especially lavas, are gla.s.sy; others are so fine grained that the crystals cannot be seen.

In places one may find veins filling cracks in the rocks, and {113} made of material deposited from solution in water. Many valuable minerals and ores occur in such veins, and fine specimens can sometimes be obtained from them.

{112}

[Ill.u.s.tration: Fold in stratified rock]

[Ill.u.s.tration: Wearing the soft and hard beds by rain and wind]

[Ill.u.s.tration: Quartz vein in rock]

{113 continued}

Sedimentary rock are formed of material usually derived from the breaking up and wearing away of older rocks. When first deposited, the materials are loose, but later, when covered by other beds, they become hardened into solid rock. If the layers were of sand, the rock is sandstone; if of clay, it is shale. Rocks made of layers of pebbles are called conglomerate or pudding-stone; those of limy material, derived perhaps from sh.e.l.ls, are limestone. Many sedimentary rocks contain fossils, which are the sh.e.l.ls or bones of animals or the stems and leaves of plants living in former times, and buried by successive beds of sand or mud spread over them. Much of the land is covered by a thin surface deposit of clay, sand, or gravel, which is yet loose material and which shows the mode of formation of sedimentary rocks.

Some rocks have undergone, since their formation, great pressure or heat and have been much changed. They are called metamorphic rocks.

Some are now made of crystals though at first they were not; in others the minerals have become arranged {114} in layers closely resembling the beds of sedimentary rocks; still others, like slate, tend to split into thin plates.

The earth's surface is continually being changed; the outcropping hard rock is worn away by wind and rain, and is broken up by frost, by solution of some minerals, etc. The loose material formed is blown away or washed away by rain and deposited elsewhere by streams in gravel bars, sand beds, and mud flats. The streams cut away their beds, aided by the sand and pebbles washed along. Thus the hills are being worn down and the valleys deepened and widened, and the materials of the land are slowly being moved toward the sea, again to be deposited in beds.

[Ill.u.s.tration: Wave-cut cliff with beach and spit built by waves and currents]

Along the coast the waves, with the pebbles washed about, are wearing away the land and spreading out its materials in new beds elsewhere.

The sh.o.r.e is being cut back in some places and built out in others.

Rivers bring down sand and mud and build deltas or bars at their mouths.

Volcanoes pour out melted rock on the surface, and much fine material is blown out in eruptions. Swamps are filled {115} by dead vegetable matter and by sand and mud washed in. These materials form new rocks and build up the surface. Thus the two processes, the wearing down in some places and the building up in others, are tending to bring the surface to a uniform level. Another process, so slow that it can be observed only through long periods of time, tends to deform the earth's crust and to make the surface more irregular. In times past, layers of rock once horizontal have been bent and folded into great arches and troughs, and large areas of the earth's surface have been raised high above sea-level.

[Ill.u.s.tration: Rock ledge rounded smooth and scratched by ice]

[Ill.u.s.tration: Sand-dune with wind-rippled surface]

At almost any rock outcrop the result of {116} the breaking-up process may be seen; the outer portion is softer, more easily broken, and of different color from the fresh rock, as shown by breaking open a large piece. The wearing away of the land surface is well shown in rain gullies, and the carrying along and depositing of sand and gravel may be seen in almost any stream. In the Northern states and Canada, which at one time were covered by a great sheet of ice, moving southward and grinding off the surface over which it pa.s.sed, most of the rock outcrops are smoothly rounded and many show scratches made by pebbles dragged along by the ice. The hills too have {117} smoother and rounder outlines, as compared with those farther south where the land has been carved only by rain and streams. Along the coast the wearing away of the land by waves is shown at cliffs, found where the coast is high, and by the abundant pebbles on the beaches, which are built of material torn from the land by the waves. Sand bars and tidal flats show the deposition of material brought by streams and spread out by currents. Sand dunes and barrens ill.u.s.trate the carrying and spreading out of fine material by the wind.

[Ill.u.s.tration: Slab containing fossil sh.e.l.ls]

[Ill.u.s.tration: Conglomerate or pudding-stone]

In many regions the beds of sedimentary rocks, which must have been nearly horizontal when formed, are now found sloping at various angles or standing on edge, the result of slow deforming of these beds at an earlier time. As some beds are more easily worn away than others, the hills and valleys in such regions owe their form and position largely to the different extent to which the harder and softer beds have been worn down by weather and by streams. The irregular line of many coasts is likewise due to the different hardness of the rocks along the sh.o.r.e.

It is by the study of the rocks and of the remains of life found in them, by observing the way in which the surface of the earth is being changed and examining the results of those changes and by concluding that similar results were produced in former times in the same way, that geologists are able to read much of the past history of the earth, uncounted years before there were men upon it.

Plants, Ferns, and Gra.s.ses

_By Dr. L. C. Corbett, Horticulturist, United States Bureau of Plant Industry_

The appearance of the blossoms and fruits of the fields and forests in any locality note the advent and progress of the seasons more accurately than does the calendar. Plants and seeds which have lain asleep during the winter are awakened not by the birth of a month, but by the return of heat and moisture in proper proportions. This may be early one year and late another, but, no matter what the calendar says, the plants respond to the call and give evidence of spring, summer, or autumn as the case may be. The surface of the earth is not flat. We have valleys and we have mountains; we have torrid and we have temperate zones. The plant life of the world has been adjusted to these varied conditions, and as a result we have plants with certain characteristics growing in the tropics at sea-level, but a very different cla.s.s of plants with {118} different habits and characteristics inhabiting the elevated regions of this same zone. It must be remembered that even under the tropics some of the highest mountains carry a perpetual snow-cap. There is therefore all possible gradations of climate from sea-level to the top of such mountains, even at the equator, and plant life is as a result as varied as is climate. Each zone, whether determined by lat.i.tude or by alt.i.tude, possesses a distinctive flora.

But alt.i.tude and lat.i.tude are not the only factors which have been instrumental in determining the plants found in any particular locality. This old earth of ours has not always been as we see her to-day. The nature we know and observe is quite different from that which existed in earlier ages of the earth's history. The plants, the trees, and the flowers that existed upon the earth during the age when our coal was being deposited were very different from those we now have. There has been a change, but, strange as it may seem, there are in some places upon the earth to-day some of the same species of plants which were abundant during the coal-forming periods. These are among the oldest representatives of the plant world now extant. Then we are told that there was a period when the north temperate zone was covered with a great ice field which crowded down as far as southern Pennsylvania and central Ohio. This naturally brought about a profound change in the location and character of the plants of this region.

There are in the Black Hills of Dakota species of plants which have no relatives anywhere in the prairie region, and no means is known by which these representatives of a Rocky Mountain family could find their way into the Black Hills, save that, previous to the ice age, this species was generally scattered over the territory, and that, during the ice age, the species was perpetuated in the hills, but was killed out between there and the Rocky Mountains where it is found in abundance. These are some of the natural reasons for the existence of varied plants in different localities. They are sufficient to explain the reason for the existence of local floras.

But nature has provided untold ways for the perpetuation as well as the dispersal of plants for the purpose of, so far as possible, enabling the plants of the world to take possession of all parts of the earth's surface. If this adjustment were complete, the plants would be practically alike all over the surface of the earth, but we have already explained why this cannot be and why we have a different flora in each zone, whether it be marked by lines of lat.i.tude or height of {120} the mountains. Plants are perpetuated by seeds, by bulbs, and by woody parts. Some seeds are highly perishable and must be sown as soon as ripe; others remain years without losing their power to produce plants. Some grow as soon as they come in contact with the soil; others must fall, be buried and frozen before they will germinate. Some plants are perpetuated by bulbs, tubers, or roots in which a supply of food material is stored away to carry the plant over a period when its above-ground parts cannot thrive owing to frost or drought. Upon the return of favorable conditions, these resting parts throw out shoots and again make the round of growth, usually producing both seeds and underground parts for the preservation of the species.

There are both wild and cultivated plants in nearly all sections which ill.u.s.trate these methods of preservation. Besides plants which have bulbs, tubers, or perennial roots, we have the large, woody plants which live many years and so perpetuate themselves, not only as individuals the same as plants with perennial roots; but they, too, as a rule, produce seed for the multiplication of their kind.