446 A . Hague — Age of the Igneous Rocks 



shaly limestone, carrying a fauna characteristic of the Middle 

 Cambrian period. These beds have been designated the Flat- 

 head formation. Above these beds, in their regular order of 

 succession, come the Upper Cambrian, Silurian, Devonian, Car- 

 boniferous, Trias, Jura, and every grand division of the Cre- 

 taceous recognized in central Montana and northern Wyoming, 

 including the Dakota, Colorado, Montana, and the Laramie 

 sandstone at the top. 



The Montana formation exhibits throughout its entire devel- 

 opment a singularly uniform sandstone epoch, the argillaceous 

 beds of the Pierre shales being poorly represented other than 

 by a great thickness of arenaceous deposits. The overlying 

 Laramie, on the other hand, although essentially a sandstone- 

 making epoch, indicates frequent and abrupt changes in its 

 sedimentation. Constantly changing beds of shales, clays, and 

 impure standstones, afford abundant evidence in their mode 

 of occurrence, that the material forming the beds was deposited 

 in a shallow sea. With the close of the Laramie sandstones, 

 the conformable Paleozoic and Mesozoic strata came to an end. 

 The entire region was again elevated above the sea. The sedi- 

 mentary beds were everywhere uplifted, and very large areas 

 tilted up at high angles. A profound orographic movement 

 took place and the region became one of mountain building on 

 a grand scale, accompanied by plication, folding, and faulting. 

 It was this movement that blocked out the mountains which 

 surround the Yellowstone Park plateau, and indeed all ranges of 

 the northern Rocky mountains. It has been designated the post- 

 Laramie movement. This orographic disturbance was doubt- 

 less coincident with the similar movements described by Mr. 

 S. F. Emmons as clearly defined in Colorado, though in the 

 northern country no unconformities such as have been noted 

 to the southward, have as yet been definitely recognized 

 between the Middle Cambrian and the Laramie, as described 

 by Mr. Emmons.* This limitation of the Laramie formation 

 to the beds at the top of the upturned conformable series of 

 sediments, is the line of demarcation first proposed by Mr. 

 Clarence King.f It is in accord with the great physical break 

 which played so important a part in the building up of the 

 northern Rocky Mountains, and brought to a close a period of 

 a distinctly marine or brackish fauna. 



In this region there are no evidences that igneous rocks 

 played any part during Paleozoic and Mesozoic times, prior to 

 the post-Laramie movement. There are no intrusive masses 

 nor interbedded flows contemporaneous with the deposition of 



* Orographic Movement in the Rocky Mountains, S. F. Emmons, Bull. Geol. • 

 Soc. Amer., vol. i, 1890. 



f U. S. Exploration of the 40th Parallel, Systematic Geology, vol. i, p. 357. 



