GEOLOGICAL SURVEY OF THE TERRITORIES. 



29 



Missouri and the Mississippi, the greater portion of these deposits to 

 form the great Yellow Marl or Loess group. That the origin of these 

 deposits dates back to the Pliocene period at least, we believe from the 

 evidence given by the organic remains; and there is other evidence that 

 points to the conclusion that these lakes continued up nearly or quite 

 to our present period. The surface is usually covered to a greater or 

 less extent with the usual drift deposit of the country, and not uufre- 

 queutly the groups of rounded bowlders are so arranged as to indicate 

 that no important changes have taken place since the waters subsided. 

 In the Yellowstone Valley, about two miles above Boteler's ranch, the 

 river has exi^osed a section of the Pliocene beds, (Fig. 1,) which reveals 



Fie. I. 







MODERN LAKE DEPOSITS CAPPED WITH BASALT. TWO MILES ABOVE BOTELER S RANXH, 

 VALLEY OF THE YELLOWSTONE. 



about 200 feet of light-gray marly sands and sandstone, passing up into 

 abont 100 feet of pebbly drift, the whole capped with a bed of basalt 

 that must have overflowed since the lakes existed in full force. A 

 little distance back ot the river, extending to the base of the mountains, 

 there is evidence in the superficial deposits that this lake continued a 

 long time after the overflow of the igneous matter. 



I shall pass rapidly over the geology of the region about the source of 

 the Gallatin, referring the reader to the excellent report of Dr. Peale, 

 who made a minute examination of Spring Cahon, Mystic Lake, and the 

 district about Mount Blackmore. 



In general terms, the Gallatin Eange is comi)osed of gneissic and quartz- 

 itic beds as a base, with a great thickness, 1,700 to 1,800 feet, of Lower 

 Silurian strata resting unconformably upon them. Above these, and 

 apparently conforming, is a thickness of 1,000 to 2,000 feet of well- 

 marked jCarboniferous rocks, mostly limestones, more or less pure ; 

 then running eastward from the Gallatin Valley and inclining at various 

 angles in the same general direction are the Jurassic, Cretaceous and 

 Goal groups, with an aggregate thickness of 10,000 to 15,000 feet. This 

 entire group of strata, which compose the Gallatin Range and its foot 

 hills, extends nearly to Shields's liiver, a distance of twenty miles in a 

 straight line. The Carboniferous limestones, which are very hard and 

 yield less readily to atmospheric influences, form, with their upturned 

 edges, the very summit of the Gallatin Eange, including Bridger's Peak, 

 Union Peak, &c. On the west side of the Gallatin Range the foot-hills are 



