396 



HISTORICAL GEOLOGY 



invertebrates which preceded them, and in turn yielded par- 

 tially to the reptiles, which began their rise near the close of 

 the era. 



Of the evolution of the plants we know much less ; but it is 

 clear that the ferns and some of the gymnosperms were the 

 prevalent types in the later Paleozoic periods. The higher 

 groups were still to be evolved. 



QUESTIONS 



1. Is it necessary to assume that salt lakes are detached parts 

 of the ocean ? Can fresh lakes ever become salt, and if so, how ? 



2. Can you suggest a reason why desert sandstones like some 

 of those in the Permian system are usually cross bedded ? 



3. In Germany a single bed of salt in the Permian system is 

 said to be more than four thousand feet thick. What does this 

 indicate ? 



4. What is the significance of the Permian system in India with 

 reference to Laplace's theory of the origin of the earth ? 



5. Although in Australia 

 the center of Permian gla- 

 ciation is not known, and 

 neither surface moraines nor 

 drumlins have been identi- 

 fied, the direction of glacial 



, movement has been deter- 

 FIG. 415. Glacial markings on a rock . , _. 



surface. mined. Can you suggest 



how from a single striated 



surface of rock (such as represented in Fig. 415) one may learn 

 whether the ice was moving toward the left or right ? 



6. It is often said that coal has been formed in tropical jungles. 

 What is the significance of the coal beds which lie between sheets 

 of glacial till in Australia ? 



