FORMATIONS AND PHYSICAL HISTORY 525 



The Comanchean of Mexico is mainly limestone, and, though 

 hut imperfectly known, has been estimated to have the extraordi- 

 nary thickness of 10,000 to 20,000 feet. Its distribution is such as 

 ID show that a large part of that country was beneath the sea. It 

 has been conjectured that the waters of the Atlantic and Pacific 

 met over the site of some part of Mexico at this time, but this is 

 uncertain. If the oceans were connected, it was probably across 

 southern Mexico, or perhaps Central America. At any rate, there 

 does not seem to have been free faunal intermigration between the 

 Gulf coast and the coast of California. 



Northern interior. The sea is not known to have extended north 

 of Kansas during the period; but clastic beds of terrestrial origin, 

 and perhaps of Comanchean age, are known at various points farther 

 north. The beds in question, the Morrison (p. 504) beds, are best 

 known along the Front Range through Colorado and Wyoming, and 

 in the Black Hills, though they probably reach northward to Mon- 

 tana. If all the beds thought to be the equivalent of the Morrison 

 are really so, the formation is widely distributed. These beds are 

 regarded by some as Jurassic, and this may be their proper classi- 

 fication. 



In Montana, Alberta, and Assiniboia, there are beds (the Koote- 

 nay and Cascade formations, etc.) similar in character to those just 

 nu-ntioned. They are mainly clastic, and contain some coal. Their 

 fossils are mostly of plants of early Cretaceous types. In Mon- 

 tana, the Kootenay formation overlies the Morrison. 



To the Morrison and Kootenay formations a lacustrine origin 

 has usually been assigned, and there is perhaps no adequate ground 

 for questioning this conclusion for some parts of the formations; 

 but the character of some of the beds and the nature and distribu- 

 tion of their fossils suggest a fluviatile origin for parts, and perhaps 

 for large parts, of the series. 



Pacific border. The system (known as the Shastan group) has 

 great development in California, where it attains its maximum 

 known thickness. It is made up of the Kiio.rcille series below and 

 the Horsetown series above. The deposits are thickest in the 

 Sacramento valley. Most of the thick system, including its basal 

 beds, bears the marks of shallow-water origin. The Shastan group 

 is represented in Oregon also. 



Where the base of the system has been observed, it is uncon- 

 formable on Jurassic rocks, or on metamorphic rocks of unknown 



