404 



HISTORICAL GEOLOGY 



Ferns continued to be common, but there were more of the 

 small varieties, like those now growing in our forests, than of 

 the tree ferns. The woodlands of the Triassic times doubtless 

 had a somber aspect not unlike that of our pine forests to-day. 

 Nor is it surprising that the purely vegetarian land animals 

 were then so little developed, when we consider that the 

 tough, fibrous leaves of palms and the resinous needles of pines 

 and similar plants are among the least palatable foods for our 

 modern cattle and wild animals. The introduction of these 

 higher animals seems to have awaited the evolution of the 

 flowering plants, particularly the grasses. 



QUESTIONS 



1. In the Humboldt Mountains of Nevada the marine Triassic 

 recks rest on metamorphosed pre-Cambrian beds. What different 

 explanations may be offered ? 



Fio. 423. Conglom- 

 erate with lava peb- 

 bles resting upon a 

 sheet of lava. 



FIG. 424. Sheet of 

 lava with secondary 

 minerals in the ad- 

 jacent shale. 



FIG. 425. A sheet 

 of lava with in- 

 cluded pieces of the 

 adjacent rock. 



FIG. 426. Sheets of lava inter- 

 bedded with layers of tuff. 



2. In the diagrams (Figs. 423- 

 426), which are the intrusive and 

 which are the extrusive lava sheets, 

 and how are the facts known ? 



3. Small bodies of copper ore have 

 been found in the Newark rocks. 

 What do you suspect as to their 

 origin ? 



4. Some of the sandstones in the 



Newark series contain abundant grains of feldspar and flakes of mica. 

 How do these rocks differ from ordinary sandstone ? Can you suggest 

 the conditions under which the two different varieties are made ? 



5. What kinds of fossils would you expect to find in the Newark 

 series, and what about their abundance ? Why ? 



