DEFINITIONS AND LAWS OF GEOLOGY.  13 



globigerina ooze, or chalky deposit, is limited to about 2,250 fathoms, and at greater 

 depths the deposit gradually passes into fine pure clay, and below 2,500 fathoms it 

 consists almost wholly of a silicate of the red oxide of iron and alumina. At 

 moderate depths shells fall upon the bottom, in perfect condition; as the depth 

 increases they become more and more brittle, and finally break up and disappear by 

 the chemical action which affects them, until, having passed through 2,500 fathoms 

 of water, nothing is left save an insoluble residue, which constitutes the red clay. 

 The simple fact of the increasing depth of the ocean gives variety to the character 

 of the deposits. But at the greatest depth to which the dredge has descended, 

 which exceeds five miles, the silicious shells of Radiolarians exist as abundantly as 

 they do in the shallower depths of the ocean. Such deposits, in the process of 

 induration, become stratified and laminated, and form calcareous, argillaceous, and 

 arenaceous or silicious rocks. 



§ 19. Animals, secreting carbonate of lime, have played an important part in 

 modifying the surface of the earth. The coral-making polyp has wrought great 

 changes, because the reef-forming genera continue the accumulation, on the same 

 spot, for centuries, and the influence of the Bryozoa, which produce only delicate 

 corals, is everywhere conspicuously engraved. There are other agents, inferior in 

 operation, affecting the surface of the earth, and all combined have served in times 

 past to deposit in water all the rocks constituting the continent of North America, 

 and to elevate the land above the seas and lakes, after such deposition, and again to 

 denude it and present it to us with its mountains and valleys as they now exist. 



§ 20. Every part of the surface of the earth has been covered with water, and 

 much of what is now dry land has been several times inundated ; and it is supposed 

 a large part, if not the whole area covered by the oceans, has, at some period of 

 time, been above the water line. The elevations and depressions have been in the 

 form of ridges, with intervening basins, in different ages of the world ; and basins, 

 existing in the same age, have been filled with deposits of different kinds and in 

 different degrees of rapidity, — some being filled with drifted materials, and others 

 with the secretions of animal and vegetable organisms. Consequently there is a 

 great diversity in the structure of the land of different continents, and they must be 

 separately investigated. The most recent deposits may be made on the most ancient 

 rocks. Cretaceous deposits may occur upon the Silurian, or Jurassic on the Devonian ; 

 hence, many difficulties are encountered in ascertaining the chronological order of 

 the strata upon each continent; and this would be utterly impossible were it not for 

 the animal and vegetable remains, which have followed the progress of time in evolu- 

 tions of type and structure in different oceanic basins, so as to furnish the means of 

 approximately parallelizing the strata. Different kinds of rocks are forming at un- 

 equal depths of the ocean, at the same time; conglomerates and sandstones in shallow 

 water and near the shores ; chalky, and slaty or shaly in deeper water, and silicious 

 farther from land and at still greater depths. Strata of the same kind are not con- 

 tinuous over large areas; but change within short distances from sandstone to shale 

 or limestone ; hence, it is never safe to trust to the character of the rock for the testi- 

 mony to prove its age. We must go to the fossils for the evidence, because it has 

 been ascertained that species did not generally live beyond a geological period, and 

 characterized different Groups of rocks, aud thus become infallible guides to the 

 order of superposition. No two periods are represented by like assemblages of fossil 



