Page images

chronological. Arr ANGEMENT OF STRATA. 195


Ideal section of the crust of the earth, showing the chronological arrangement of the strata.





. The Wealden.
. The Oolite.

. The Lias.
. The Saliferous, consisting of

Alluvial or modern deposits. a. 2. do. do. Overlaying the coal Tertiary formations. formation. Cretaceous system, compris- b. 1. Granite veins in Granite. ing the chalk, with & with- b. 2. do. do. passing through porout flints, chalk marl, galt phyry, gneiss, & mica schist. or blue clay, Shanklin sand. b. 3. do. do. Overlaying Grauwacke. c. Dyke of Trap, passing thro' grauwacke and the laminations of marine limestones, forming basaltic columns. c. 1. do. do. intersecting, and

New Red Sandstone, Mag-
nesian Limestone.

. The Carboniferous System,

namely, the Coal measures,
the Mountain limestone.

. The Devonian, or Old Red

overlaying an older dyke of porphyry.

c. 2. do. do. overlaying the oolite


Sandstone. c. 3. do. do. overlaying the chalk being lava of the extinct volcanoes of the Tertiary period.

d. Modern lava.

The Silurian, consisting of
marine limestones, shales,
calcareous flags.
The Cambrian.
Mica Schist.

The Gneiss.
Granitic System.
Volcanic Rock.

e. Cave in magnesian limestone.
f. Metallic veins in granite.
g. Extinct volcano.
h. Artesian well.

a. 1. Dyke of Porphyry in gran

ite, passing through & over-
laying gneiss.

The order of succession which we have given has been determined by a series of the most patient investigations, and from an immense accumulation of facts, collected by able observers in all parts of the globe. On page 194 we have given a diagram showing their order. It will be perceived that some parts of the representation are necessarily exaggerated. Commencing at the left we have the hypogene, or underlying rocks, unstratified and stratified; then the primary and secondary fossiliferous, and also the tertiary, lastly the volcanic. The alluvium we have placed in various situations, overlaying the older rocks,


Aqueous Causes of Change.

“The rivers swell,
Of bonds impatient. Sudden from the hills,
O'er rocks and woods, in broad, brown cataracts,
A thousand snow-fed torrents shoot at once ;
And where they rush, the wide resounding plain
Is left one slimy waste.” Thomson.

In the preceding chapter we have given a general view of the arrangement of the strata which compose the crust of the earth. We now proceed to consider the changes at present going on in the organic and inorganic kingdoms of nature. We will thus be better prepared to admit that the fossil remains which occur everywhere in the stratified rocks, and that the stratification of those rocks, are results of laws now in full operation, but exerted through a period of years, it would be utterly vain to attempt to estimate. In the present chapter, which presents us with a subject which of itself might form a volume, we can only hastily glance, at some of the most active causes of change now in operation, those who desire to learn more will find ample information in the writings of Lyell, Mantell, Buckland, and other well known geologists. Although from the very nature of the case, geology is somewhat a speculative science, since it takes into consideration the changes and vicissitudes which the earth has undergone, during ages so remote, that the mind can with difficulty conceive of the lapse of time past, and endeavors to explain them by the application of laws now in action, but whose silent operation is unheeded by the great mass, yet it at the same time presents us with the noblest views of the material universe; and the philosophic mind, in reviewing ever so cursorily, the traces of the past, cannot fail to be struck with the harmony of the material world. Everything around us is in a most active state of change, literally speaking there is no such thing as rest. Every operation of nature, however minute and familiar, the heat and the cold, the moisture and the drouth, the warmth of summer and the frosts of winter, the snow and the ice, nay, every drop of rain that falls from the atmosphere, performs its share in displacing and renewing the solid crust of the earth, and contributes its alloted portion in carrying on the great work of universal metamorphosis and change. The great agents of change in the inorganic world may be divided into two classes, the aqueous and the igneous. To the aqueous belong rivers, torrents, springs, currents, and tides ; to the igneous volcanoes and earthquakes. Beside these we may enumerate the agency of the atmosphere, which is partly mechanical, and partly chemical; and vital action. We shall consider these several agents of change in order, and see their present effect in changing the sea to land, and land to sea; in excavating valleys and destroying hills; in the transition of dry ground to marshes, and the reverse; the occurrence of earthquakes and their phenomena; the uniting of islands with main lands, and insulation of peninsulas. In this manner, although we may not be able to comprehend entirely the degrading and elevating causes above enumerated, yet we will see abundant means for the conversion of the soil upon which we now tread, from the bed of an ocean to dry land; we shall see how wood has been changed into stone, and plants and fishes imbedded in solid rock. We shall first consider the action of running water. The heated atmosphere which sweeps over the vast ocean and the surface of the earth, absorbs and carries with it an immense amount of aqueous vapor, to be again deposited when the air is cooled, in the form of clouds, mist or rain. A large amount of this moisture is deposited upon mountains and elevated lands, and thus the more elevated regions become perpetual reservoirs of water, which flows down in gentle streams and rivers, irrigating the plains below. At the first glance we might suppose the amount of water carried up into the air by evaporation was of too trifling a nature to be instrumental in effecting any great mechanical change, but a moments reflection

Action of RUNNING WATER. 199

will convince us that the amount is almost beyond estimate. All the rivers on the face of the earth are constantly pouring their waters into the sea, and yet its level is not affected in the slightest degree, hence we infer that the quantity of mositure evaporated from the surface is exactly equal to the sum of all the rivers of the world. If the evaporation and restoration of the waters were all the effect which is produced by the agencies just doscribed, little change would be accomplished upon the face of the country over which the waters might flow in their passage to the sea. But in the more elevated tracts of country, the atmosphere acts powerfully upon the soil, and by the influence of heat and cold, by dampness and dryness, and of frost and rain, loosens the most coherent masses and disintegrates the solid rocks. The mountain streams flow down more or less charged with earthy matter, worn from the soil and rocks over which they flow. In their passages toward the sea, sometimes over an immense tract of country, they often unite and pour their waters along with almost irresistible fury. The solvent power of the water assists very materially in degrading the rocky channels through which it flows, and acts powerfully on the alkaline and calcareous elements of the soil, and especially when it holds carbonic acid in solution, which is almost always the case. When the earthy matter and pebbles are thus intermingled with running water, a new mechanical power is gained, by the attrition as they are borne along, thus sapping and gradually undermining high banks and rocks, until at length the overhanging mass is precipitated into the current and swept away by its waters. In this manner, islands are cut off from the main lands, and shoals, and rich earthy deposits called deltas are formed at the mouths of rivers. There is nothing so very remarkable in the power of currents to transport even heavy masses of stone, for we must remember that the specific gravity of water is much greater than air, and a stone immersed in a stream will loose about half its weight, and many of the lighter particles of the soil will almost float. Sir George Staunton estimated that the quantity of sediment borne down by the Yellow River in China, in a single day, was equal to forty-eight millions of cubic feet, and late observations

« PreviousContinue »