GEOLOGICAL HISTORY OF THE YKLLOWSTONE PARK. 135 



coniu'cts the Absarokas with tlic Snowy Itaiijie. This Snowy Ranj;v 

 shuts in the Park on the north, and is an c.iu;ill.\' roni;h r<^j;iou of conn 

 tiy, with ek'N'atcd mount -liu luasscs coNcred with snow tlio jiieater part 

 ot the year, ;is tlie nanu' wouhl indicate. Only tlie southern sh)pes, 

 wliieli rim in the I'ark I'egion, come within the limit of our investiga- 

 tion. Here the roidcs are mainly granites, gneisses, and schists, the 

 sedinu-ntary beds, for the most part, referable to the pre-( 'and)rian 

 series. 



The Galhitin Range incloses the Park on the north and northwest. 

 It lies directly west of the Snowy, only separated by the broad valley 

 of the Yellowstone River. It is a range of great beauty, of diversified 

 forms, and varied geological problems. Electric Peak, lu the extreme 

 ncH'th western corner of the Park, is the culminating ])oint in the range, 

 and affords one of the most extended views to be found in this part of 

 the country. Archean gneisses form a prominent mass in the range 

 over which occur a series of sandstones, limestones, and shales, of Pale- 

 ozoic and Meso/oic age, representing Cambrian, Silurian, Devonian, 

 Carboniferous, Trias, Jura, and Cretaceous. Immediately associated 

 with these sedimentary beds, are large misses of intrusive rocks, 

 which hav^e played an important part in bringing about the present 

 structural features of the range, Tiiey are all of the andesitic t^'jie, 

 but sliowing considerable range in mineral composition, including 

 ]>yroxene, hornblende, and hornblende mi(;a varieties. These intrusive 

 masses are f)unil in narrow dikes, in immense interbedded sheets 

 forced between the different strata, and as laccolites, a mode of occur- 

 rence first described from the Henry Mountains in Utah, by Mr. G. K. 

 CJilbert, but now well recognized elsewhere in the northern Cordillera. 



We see then tiiat the Absarokas rise as a formidable barrier on the 

 eastern sid(^ of tiie Park, the Gallatins as a steep mural face on the 

 west side, while the other ranges terminate ai>ru]>tly, rimming in the 

 Park on the north and south, and le iving a depressed region not unlike 

 llu; parks of Colorado, only covering a. more extended area with a rela- 

 tively deeper basin. The region has been one of profound dynamic 

 action, and tlic center of mountain building on a grand scale. On the 

 accompanying map of the Yellowstone Park, which shows the position 

 of the princii)al o!))ects of interest, the relations of the ranges to the 

 plateau are clearly indicated. 



It is not my purpose at the present time to enter u]>()n the details of 

 ge:)logical structure oftln^se ranges, each offering its own special study 

 and field of investigation. My desire is simi)ly to call your attention 

 to their general features and mutual relations. So far as their age is 

 concerned, evidence goes to show that the action of upheaval was con- 

 temporaneous in all of them, and coincident wMth the powerful dy- 

 namic movements wliich uplifted the north and south ranges, stretch- 

 ing across (Colorado, Wyoming, and Montana. This dynamic move- 

 ment blocked out, for the most part, the Rocky Mountains, near the 



