12 INTRODUCTION. 



prehend the earth's history. The science, therefore, is a historical science. 

 It finds strata of sandstone, clayey rocks, and limestone, lying above one 

 another in many successions; and, observing them in their order, it assumes, 

 not only that the sandstones were made of sand by some slow process, 

 clayey rocks of clay, and so on, but that the strata were successively formed; 

 that, therefore, they belong to sxiccessive periods in the earth's past ; that, con- 

 sequently, the lowest beds in a series were the earliest beds. It hence infers, 

 further, that each rock indicates some facts respecting the condition of the 

 sea or land at the time when it was formed, one condition originating sand 

 deposits, another clay deposits, another lime, — and, if the beds extend over 

 thousands of square miles, that the several conditions prevailed uniformly 

 to at least this same extent. The rocks are thus regarded as records of 

 successive events in the history, — indeed, as actual historical records ; and 

 every new fact ascertained by a close study of their structure, be it but the 

 occurrence of a pebble, or a seam of coal, or a bed of ore, or a crack, or 

 any marking whatever, is an addition to the records, to be interpreted by 

 careful study. 



Thus every rock marks an epoch in the history ; and groups of rocks, 

 periods ; and still larger groups, eras or ages ; and so the eras which reach 

 through geological time are represented in order by the rocks that extend 

 from the lowest to the uppermost of the series. 



If, now, the great beds of rock, instead of lying in even horizontal 

 layers, are much folded up, or lie inclined at various angles, or are broken 

 and dislocated through hundreds or thousands of feet in depth, or are 

 uplifted into mountains, they bear record of still other events in the great 

 history ; and should the geologist, by careful study, learn how the great 

 disturbance or uplifting was produced, and succeed in locating its time of 

 occurrence among the epochs registered in the rocks, he would have inter- 

 preted the record, and added not only a fact to the history, but also its 

 explanation. The history is, hence, a history of the upturnings of the 

 earth's crust, as well as of its more quiet rock-making. 



If, in addition, a fossil shell, or coral, or bone, or leaf, is found in one of 

 the beds, it is a relic of some species that lived when that rock was forming; 

 it belongs to that epoch in the world represented by the particular rock 

 containing it, and tells of the life of that epoch ; and, if numbers of such 

 organic remains occur together, they enable us to people the seas or land, to 

 our imagination, with some of the kinds of life that belonged to the ancient 

 epoch. 



Moreover, as such fossils are common in a large number of the strata, 

 from the lowest containing signs of life to the top, — that is, from the oldest 

 beds to the most recent, — by studying out the characters of these remains 

 in each, we are enabled to restore to our minds, to some extent, the popula- 

 tion of the epochs, as they follow one another in the long series. The strata 

 are thus not simply records of moving seas, sands, clays, and pebbles, and 

 disturbed or uplifted strata, but also of the living beings that have in 



