MESOZOIC ERA: THE AGE OF REPTILES 513 



lence of sandstones, with only occasional limestone beds, shows that 

 the sea was a shallow one. Since the fossils of some of the beds are 

 of marine species, closely resembling those of Siberia rather than those 

 of California of the same age, we must suppose that a mediterranean 

 sea was connected at the north with the ocean, and that a long land 

 barrier separated it from the Pacific. After a comparatively short 

 existence this great bay was drained by elevation of the land ; and its 

 site was covered in the southern portion by a widespread, continental 

 formation (Morrison) which contains skeletons of dinosaurs and other 

 reptiles and a few mammalian remains. Because of the absence of 

 marine fossils, it is not yet certain whether these beds are late Juras- 

 sic or early Cretaceous, or whether the lower portions belong to the 

 earlier period and the upper to the later. 



Mountain Forming in the West. — In the west, where the Sierra 

 Nevada Mountains now stand, Jurassic sediments derived from 

 extensive lands on the east had been accumulating in a great subsiding 

 trough (geosyncline), until they had attained a maximum thickness of 

 five or six thousand feet. The sediments deposited in this trough, 

 including Triassic, Jurassic, and Paleozoic, attained the enormous 

 thickness of nearly 25,000 feet. Near the close of the period, 

 this huge accumulation of sediments began to yield to great lateral 

 compression and was folded and upheaved into the first Sierra 

 Nevada Mountains, which perhaps rivaled in height any in existence 

 to-day; they may, however, have been eroded almost as rapidly as 

 they rose and therefore never have reached a great elevation. Dur- 

 ing the folding great quantities of igneous rocks, especially granites, 

 were forced into the folded sediments, forming upon cooling batho- 

 liths and stocks. The muds and sands in the neighborhood of the 

 intrusions were also changed to schists and other metamorphic 

 rocks ; and even at a distance shales were metamorphosed to slates. 

 At about the same time, the Coast Ranges, the Cascades, and farther 

 north the Klamath Mountains began their growth. It should not 

 be inferred from the above that the present height of the Sierra 

 Nevadas was the result of these movements : the Sierra Nevadas 

 of to-day, as will be explained later, are the result of a great fault 

 on the east, which occurred at a much more recent date. 



Jurassic of Other Continents. — The greater portion of Europe 

 and Asia was above the sea during the earlier part of the period, but a 

 progressive submergence soon began, culminating in the Upper 

 Jurassic, at which time the two continents were traversed by straits 



