406 ABSTRACTS 



The oldest rocks of this region are granites, gneisses, and schists 

 regarded as of Archaean age. They occur in all the mountain uplifts 

 that encircle the park, but are unknown in the central portion. Around 

 these ancient continental land masses there was deposited a conform- 

 able series of sandstones, limestones, and shales extending from the 

 time of the Middle Cambrian, the lowest beds exposed, through the 

 Upper Cambrian, Silurian, Devonian, Carboniferous, Juratrias, and 

 Cretaceous, including the Laramie sandstone. Nearly all these great 

 divisions of Palaeozoic and Mesozoic times are characterized by a typ- 

 ical fauna. 



With the close of the deposition of the Laramie sandstone the con- 

 formable series of sediments came to an end. The entire region was 

 elevated above the sea, the movement being accompanied with plica- 

 tion and folding of strata. This primary orographic uplift which 

 blocked out the main ranges of the northern Rocky Mountains, has 

 been designated the post-Laramie movement. 



Tertiary sedimentary rocks occupy only small areas in the park, 

 the greater part of the region being covered by extensive flows of lava. 

 A heavy mass of coarse conglomerate, designated the Pinyon con- 

 glomerate, has been referred to the Eocene ; and Pliocene conglomerate 

 and coarse sands are well exposed in the escarpments of the Grand 

 Canyon. 



Volcanic energy, which has played so great a part in the geolog- 

 ical development of the country, was connected with the post-Laramie 

 movement and followed closely upon the elevation of the mountains, 

 and the accompanying dislocation and compression of strata. The 

 eruptive masses, in forcing their way upward, sought egress along 

 lines of least resistance, or wherever strain has been greatest in the 

 crumpled sediments. Volcanic outbursts continued on a grand scale 

 throughout Tertiary time. 



During the Eocene and Miocene periods enormous volumes of 

 fragmental ejectamenta, largely composed of andesitic and basaltic 

 breccias, were thrown out. The Absaroka Range was almost wholly 

 built up of volcanic material. Evidence of this long-continued action 

 is shown in the well-preserved fossil floras of Eocene and both Lower 

 and Upper Miocene age. The famous fossil forests of the Yellowstone 

 are of Miocene age. After a period of great erosion the depressed 

 basin lying between the encircling ranges was transformed into the 

 present Park plateau by the extravasation of immense flows of rhyolite 



