368 HISTORICAL GEOLOGY 



The rivers and other land waters supported a variety of fishes 

 and mollusks. Vegetation was luxuriant in favorable places 

 and included trees as well as the lowlier growths. The trees 

 of this period were not, however, like those of the present day. 

 Most of them were relatives of the ferns. Insects and their 

 allies have been found hi some numbers. Considering the 

 small chance of preserving such delicate creatures in the rocks, 

 it would not be reasonable to expect many fossils. 



QUESTIONS 



1. Why is the present distribution of the Devonian system less 

 than it was originally ? 



2. Can you suggest why the outcrops of the Devonian system 

 are usually narrow bands ? 



3. Oil is found in Devonian strata in certain parts of eastern 

 United States. It is usually concentrated beneath anticlines. A 

 well piercing the fold usually encounters first natural gas, deeper oil, 

 and still farther down water. Can you suggest why there should 

 be this arrangement ? 



4. Why should the wells obtain more oil from sandstone than 

 ^_ from shale ? 



5. An instance is known of the 

 occurrence of black mud containing 

 teeth and plates of Middle Devonian 

 fishes in cracks exposed in a quarry 

 in the Niagara limestone (Fig. 387). 

 Can you suggest an explanation? 

 FIG. 387. Cracks in the Ni- Q Under what conditions will two 



agara limestone filled with faunag di and beCQme legs d 



black mud and fossils. 



less like each other? 



7. Why should the fossils of the Chemung formation be less 

 like those of the Catskill beds, which are of the same age, than 

 like those of the Hamilton formation, which is distinctly older? 



8. Can you suggest why crinoids and corals are rarely found 

 in the Oriskany formation? 



9. Why is it not so easy to use the small divisions of geologic 

 time in widely separated countries, as it is to use the larger divi- 

 sions, such as eras ? 



10. When a local and a cosmopolitan fauna are permitted to 

 mingle, because of some geographic change, which of the two usually 

 exerts the stronger influence on the new fauna thus formed, and why ? 



