432 HISTORICAL GEOLOGY 



haps less striking, changes ; and when the next era opened, 

 all the large groups, except the birds and the mammals, had 

 nearly reached their modern condition. 



QUESTIONS 



1. In the drier parts of South Dakota the Cretaceous shales 

 contain many little lenses of limestone. These now stand out as 

 conical hills, known as "tepee buttes" from their resemblance to 

 an Indian tent. If the climate of this region were moist and the 

 surface densely forested these buttes would probably not be formed. 

 Why? 



2. The Dakota sandstone is exposed along the flanks of the 

 Rocky Mountains in sharp ridges, locally known as "hogbacks." 

 What does this tell about the character of the formation? 



3. Some of the Cretaceous chalk is interbedded with layers of 

 sandstone. What does this indicate about the depth of water in 

 which the chalk was formed ? 



4. In some of the Cretaceous beds sticks of wood with charred 

 ends have been found. What inference is suggested ? 



5. Chalk consists largely of the shells of protozoans. What are 

 the habits of these animals? Under what conditions do they suc- 

 ceed in forming a deposit of chalk? 



6. About the Black Hills of South Dakota the Laramie beds 

 appear to contain no material derived from the Paleozoic group, 

 while the Tertiary beds which lie upon the Laramie are largely 

 composed of such debris. How may this be explained ? 



7. Bees, butterflies, and many other insects of like habits have 

 not been found in rocks older than Cretaceous. How may this fact 

 be related to evolution among the plants? 



