THE JURASSIC PERIOD 413 



jaws were conical teeth like those of a lizard, and its long tail 

 had bones out to the very tip. The fingers of the front limbs 

 (or wings) were still free and distinct, whereas in all modern 

 birds they have entirely grown together into a single bone, 

 to which the feathers are attached. Its characteristics, then, 

 are essentially intermediate between those of reptiles and of 

 birds, and seem to indicate, with more than usual certainty, 

 that the birds are the direct descendants of some one of the 

 earlier reptiles. 



QUESTIONS 



1. The faulted lava beds of late Tertiary age in Oregon and 

 Nevada now stand out as high mountain ranges, while those in 

 New Jersey make very low mountains, or hills. Can you explain ? 



2. What does the thin shale and limestone formation of Jurassic 

 age in northwestern United States tell us about the Rocky Moun- 

 tains in that period ? 



3. There are many lava flows interbedded with the Jurassic 

 shales in California, and the shales contain marine fossils. From 

 this what do you infer as to the conditions at that time and place ? 



4. Under what conditions does molten lava form granite? 



5. Why are granites the commonest rocks in batholiths ? 



6. How does it happen that many batholiths of granite are ex- 

 posed at the surface to-day ? 



7. In what kind of a deposit should the most delicate fossil be 

 most perfectly preserved? 



B. & B. GEOL. 24 



