REVIEWS 201 



and late Ordovician periods are represented by relatively thin forma- 

 tions, chiefly of limestone. The Ordovician is found only in the 

 northern part of the state, and the Devonian, so far as now known, 

 only in the northwestern. As elsewhere in this part of the United 

 States, the Devonian seems to rest conformably on the Ordovician. 

 The Carboniferous is more fully represented than the preceding sys- 

 tems. The Lower Carboniferous is found only in the northern half 

 of the state, while the Upper is more widespread. Limestone is the 

 dominant rock. The Permian occurs in the Laramie, Big Horn, and 

 Wind River mountains. It has but slight thickness, 200 feet or so, 

 but has the fauna characteristic of the period. 



The Mesozoic systems are much more fully represented, being in 

 the aggregate 20,000 to 30,000 feet thick. They are in general con- 

 formable on the Paleozoic. 



The Triassic system is represented by the Red Beds, which are 

 gypsiferous and without fossils. The Jurassic system is represented 

 by a marine division, the Shirley formation, overlaid by a fresh-water 

 division, the Como formation. Both formations are referred to the 

 later third of the Jurassic period. No special reasons are given for 

 assigning the Como to the Jurassic, rather than to the Lower Cre- 

 taceous. This formation is said to have covered most, if not all of the 

 state, and its character is such as to indicate marshy and lacustrine 

 conditions. 



The Cretaceous formations are the most extensive in the state. 

 They cover about 50,000 square miles, and the thickness is 20,000 to 

 25,000 feet. The following formations are present: The Dakota, 

 Bear River, Fort Benton, Niobrara (which contains some chalk), the 

 Fort Pierre and Fox Hills formations, which together are, at the 

 maximum, something more than two miles thick, and the Laramie, 

 which has a thickness of about one mile. The Montana division 

 (Fort Pierre and Fox Hills) contains some oil and coal, and the Lara- 

 mie much coal. The Fort Union beds are also placed with the Cre- 

 taceous, with a question. 



The areas of the Tertiary rocks are characterized by the Bad Land 

 topography. The Eocene is represented by the Bridger, Green 

 River, and Wasatch beds ; the Oligocene by the White River beds : 

 the Miocene by the Loup Fork. The Eocene beds have an aggregate 

 thickness of 3500 feet, the White River of 1500, and the Loup Fork 

 of 500. Pliocene beds are not known. 



