March 24, 1899.] 



SCIENCE. 



427 



coniferous forest, and the greater part of 

 the forests lying east of the Yellowstone 

 Park belong to the Yellowstone timber re- 

 serve, the first of the forest reservations set 

 aside by proclamation of the President under 

 the Act of Congress approved March 1, 1891. 

 Eightly to understand the true position 

 of this volcanic area it is necessary to re- 

 view briefly the geological history of the 

 surroiinding region before the piliug-uf) of 

 the eruptive material. The Absarokas are 

 hemmed in, both to the north and to the 

 south, by high ranges with approximately 

 east and west trends. On the north are the 

 Beartooth Mountains, presenting a broad 

 elevated Archean mass culminating in some 

 of the highest peaks to be found in Montana; 

 while to the south are the Owl Mountains, 

 consisting of an Archean nucleus capped 

 and for the most part concealed by an arch 

 of Paleozoic beds highly inclined along the 

 outer" edges. Between these two ranges 

 lies a depressed basin, and resting uncon- 

 formably upon the Archean are sediments 

 of great thickness, derived in large part 

 from the earlier continental areas. 



These sediments, slowly deposited through- 

 out a long period, represent nearly all the 

 great divisions of Paleozoic and Mesozoic 

 time. Beginning with the Cambrian, in 

 their order of sequence, come the Silurian, 

 Devonian, Trias, Jura and all the epochs 

 of the Cretaceous recognized in Wyoming 

 and Montana including the Dakota, Colo- 

 rado, Montana and the Laramie standstone 

 at the top, with its frequent fluctuations of 

 sea level, foreshadowing changes in the de- 

 velopment of the pre-existing continental 

 area. 



With the close of the Laramie sandstone 

 the long-continued deposition of Mesozoic 

 and Paleozoic sediments finally came to an 

 end. In this i-egion unconformity of sedi- 

 ments by deposition has not as yet been 

 recognized, and in this sense alone they 

 may be said to be conformable from Middle 



Cambrian time to the summit of the Lara- 

 mie. Stupendous orogenic movements 

 took place, and the surrounding countrj' 

 became one of mountain building on a grand 

 scale, accompanied by plication, folding 

 and faulting. The evidence all points in 

 one direction — that this uplifting was con- 

 temporaneous in all the ranges of the 

 northern Eocky Mountains. For this 

 reason, and owing to its great geological 

 significance, being one of the most impor- 

 tant in Rocky Mountain geological history, 

 the uplifting has been designated as the 

 post-Laramie movement. 



Along the west side of the Absarokas, 

 and lying within the Yellowstone Park, ex- 

 tend north and south ridges of faulted and 

 crumbled strata consisting mainly of highly 

 inclined Cretaceous sandstone, the Laramie, 

 nearly 10,000 feet above present sea level. 

 From this ridge region eastward for fifty 

 miles stretches this broad volcanic mass, 

 finally dying out upon the plain over which 

 the earliest lavas spread, resting on hor- 

 izontal sandstones at an elevation of about 

 6,000 feet above sea level. After a very 

 considerable erosion of the uplifted Meso- 

 zoic continental land area began the earliest 

 of these volcanic eruptions, which later 

 displayed such marvelous energy over this 

 entire region of country, and which were 

 closely related to the post-Laramie move- 

 ment. This eruptive material, forcing its 

 way upwai-d, followed lines of least resist- 

 ance along or near planes of faulting, or 

 wherever the strain had been greatest upon 

 the weakened strata. 



The Absaroka Range was formed by the 

 piling-up of successive accumulations of 

 volcanic ejectamenta, with occasional inter- 

 bedded flows of lava, burying evei-ything 

 beneath them to a depth of several thous- 

 and feet. Volcanic breccias, agglomerates 

 and extrusive lavas, or those that have 

 been poured out and cooled near the sur- 

 face, constitute the bulk of the mountains. 



