EVIDENCE OF FOSSIL REMAINS 89 



up and overlie the sedimentary strata formed regularly 

 in other ways and at other times. The volcanoes of the 

 Java region alone have thrown out at least 100 cubic 

 miles of lava, cinders, and ashes during the last 100 years 

 - twenty times the bulk of the materials discharged 

 into the Gulf of Mexico by the Mississippi River in the 

 same period of time. 



From these and similar facts, the naturalist finds how 

 agencies of the present construct new rocks and alter 

 the old ; and so in the light of this knowledge, he pro- 

 ceeds with his task of analyzing the remote past, confi- 

 dent that the same natural forces have done the work of 

 constructing the lower geological levels because these 

 earlier products are similar to those being formed to-day. 

 After learning this much, he must immediately under- 

 take to arrange the strata according to their ages. This 

 might seem a difficult or even an impossible task, but 

 the rocks themselves provide him with sure guidance. 



Wherever a river has graven its deep way through an 

 area of hard rocks, as in the case of Niagara, the walls 

 display on their cut surfaces a series of lines and planes 

 showing that they are superimposed layers formed 

 serially by deposits that have differed some or much at 

 different times according to the circumstances control- 

 ling the erosion of their constituent particles. A layer 

 of several feet in thickness may be composed of com- 

 pact shale, while above it will be a zone of limestone, 

 and again above this another layer of shale. Successive 

 strata like these, where they are parallel and obviously 

 undisturbed, are evidently arranged in the order of their 



