88 Evolution of the Earth. 



Fossil Remains in the Sedimentary Rocks. Thus 

 far we have spoken only of the soil and gravel, the sand, 

 mud and pebbles of which the stratified rocks are composed. 

 They exhibit, however, other features of great interest to 

 the geologist and to the student of evolution. We have 

 seen how, in the section of material accumulating by a 

 brook-side, or in wayside pools after a long and heavy rain, 

 we not only find the mud and sand and gravel of which it 

 is composed, but also dry leaves, and sticks and debris of 

 various kinds. We have seen also how the impressions of foot- 

 prints, rain-drops and ripples are left in the sand and pre- 

 served in stone. The same result, on a much grander scale, 

 is observable in many of our stratified rocks. If you 

 search along the bed of a shallow creek in Chautauqua 

 County or elsewhere in the western part of this State, until 

 you find or can dislodge a loose piece of shale, or stratified 

 rock formed from the mud deposited in the bed of the creek, 

 and observe it closely along its line of natural cleavage, 

 you will find there the clear impressions of beautiful ferns 

 or the shells of aquatic animals, sometimes complete pet- 

 rifications of nuts and other remains of vegetable or animal 

 life. These are perhaps of comparatively recent formation ; 

 but in the stratified rocks along the mountain ranges in the 

 interior of continents, which centuries ago were formed at 

 the bottom of the ocean, we may discover the fossilized re- 

 mains of distant ages, and trace thereby the evolution of 

 life in far away geological periods. These remains, though 

 mainly, are not exclusively those of marine animals ; they 

 include the remains of sea-vegetation, and vegetable pro- 

 ducts which grow upon the land are sometimes floated out 

 to sea, and dropped among the sedimentary deposits which 

 are continually collecting in its bed. These fossil remains, 

 as you will readily understand, are of the utmost impor- 

 tance in enabling the naturalist to trace the early history of 

 the development of vegetable and animal life upon the 

 planet. By observation of them, the relative age of each 

 geological stratum may frequently be determined. 



The Formation of Organic Rocks. This is not the 

 only curious and wonderful result of the study of the strat- 

 ified rocks. Sometimes we find that organic material is 

 deposited in such quantities as to form strata of great thick- 

 ness, composed mainly or exclusively of this material, which 

 are ultimately solidified into rocks. Some rocks are, there- 



