130 INTRODUCTION TO EVOLUTION 



destroyed by forces operating on the rocks in which they are embedded. 

 It will be evident that, even when fossils are in the most favorable condi- 

 tion possible, much experience and knowledge of anatomy, of both liv- 

 ing and extinct animals, are necessary to enable the investigator to inter- 

 pret correctly the portions of animals remaining for his study. 



"Pages" in the Geologic Record 



We have seen that reading the geologic record consists of placing correct 

 interpretation upon a variety of fossil remains of animals previously exist- 

 ing on the earth. But if the history of life on earth is to be understood, the 

 separate events comprising that history must be arranged in correct se- 

 quence and relationship. Printed records are firmly bound so that the 

 pages follow one another in correct sequence to provide the reader with a 

 coherent and connected account. But in the geologic record, how are we 

 to tell which is "page 1," which "page 2," which "page 3," and so on? 



The "leaves" comprising the geologic record "book" are layers of rock, 

 called strata. A stratum consists of more or less sohdified material which 

 was originally deposited by a carrying agent such as water or air. By far 

 the larger proportion of the known strata was deposited on what was then 

 the floor of shallow extensions of the sea. In such shallow seas multitudes 

 of animals live, many of them species with calcareous shells or skeletons. 

 As generation follows generation the shells and skeletons settle to the bot- 

 tom to form a layer of ever increasing thickness. As time goes on the 

 deeper portions of this deposit are subjected to the pressure of overlying 

 portions, with the result that the deposit becomes more or less solidified 

 and consolidated into rock — limestone in the present example. If a river 

 empties into this region of the shallow sea, deposits of material transported 

 by the river mingle with the remains of marine animals. The river brings 

 the products of erosion of the neighboring land, such as silt and clay, and 

 mingled with them the remains of aquatic and terrestrial animals, par- 

 ticularly when the river is at flood stage. So through hundreds and thou- 

 sands of years the sediment accumulates and gradually becomes con- 

 solidated into sedimentary rock. 



Geologic changes in that region of the earth may eventually result in a 

 change both in the animals inhabiting that section of shallow sea and in the 

 materials being brought down by the river. The deposit following such a 

 change will, naturally, be of somewhat different nature from the deposit 

 formed before the change. Such changes account in part for the stratified 

 or "layer-cake" appearance of deposits, so evident in such places as the 



