THE JURASSIC PERIOD 221 



Thickness of the Jurassic. — The thickness of the system in 

 California does not appear to be over 2000 feet, while in western 

 Nevada 5000 to 6000 feet of limestones, and slates are reported. 

 In Alaska a maximum thickness of 10,000 feet has been found. 

 Throughout the western interior the thickness never appears to be 

 great, usually not more than a few hundred feet. 



Igneous Rocks. — Tremendous bodies of granite have been 

 intruded into rocks as young as the late Jurassic (Mariposa) slates, 

 and though the date of the intrusion is not certainly known, it 

 very likely occurred toward the close of the Jurassic period and as 

 an accompaniment of the Sierra Nevada Revolution (see below). 



Physical History 



Earlier Jurassic in the West. — During this time nearly all of 

 North America north of Mexico was a land area. Marine waters 

 spread over western Nevada, most of California, the western 

 coast of Oregon, and southern Alaska. The western interior region 

 received only continental deposits, probably including some "Red 

 Beds" and gypsum in moderate amounts. 



Later Jurassic in the West. — In the latter part of the period 

 western Nevada probably was not covered by marine waters, but 

 the other Pacific Coast regions above mentioned were. At this 

 same time an important change took place in the western interior 

 region by a transgression of the sea from British Columbia south- 

 ward over the Rocky Mountain district to northern Arizona. The 

 accompanying map (Fig. 133) shows the condition of the continent 

 at that time. Eastern Mexico appears to have been largely sub- 

 merged also. 



Eastern North America in the Jurassic. — No Jurassic strata 

 now occur in the eastern two-thirds of North America and we have 

 no evidence that any ever were deposited there, hence that vast 

 area was dry land undergoing erosion during the whole period. 

 The period was ushered in by a slight upwarping of the Atlantic 

 border accompanied by some faulting and tilting, particularly of 

 the Triassic (Newark) rocks, as shown in Fig. 120. That this up- 

 lift actually occurred, and that the Jurassic period in the eastern 

 United States was a time of extensive erosion, is well established, 

 because the whole Atlantic seaboard, including the tilted and 

 faulted Triassic strata, was worn down toward the condition of a 



