GEOLOGICAL SURVEY OF THE TERRITORIES. 65 



erson Fork, where the South Bowlder Eauge is cut off by a narrow 

 syucliiial wliich forms the valley of the Jefferson. The limestones incline 

 away trom the north' end of the South Bowlder liange, but the greater 

 portion, including all the lofty peaks, is composed of granitic rocks. 



The divide which separates the basin which we have just described 

 from the lower basin about the Three Forks is composed almost entirely 

 of metamorphic strata. Here and there we observe an outflow of igne- 

 ous matter, but very seldom. In the ravines and depressions, as it were 

 filling up the inequalities of the surface of the metamorphic rocks, are 

 the lake-deposits, which show so clearly that at a modern period the 

 Madison Basin was connected by water w-ith the entire valley of the 

 Three Forks, even far below their junction. As the waters subsided so 

 as to expose this granite-divide, they gravitated toward that portion of 

 the basin where they now tlow through the granite-cailon. What cai/sed. 

 the waters to wear out the p"resent channel is not obvious at this time. 

 The ridge or divide may have been lower at that point or there may 

 have been a slight fissure which determined its choice. 



In following- up the channels of some of the little streams that flow 

 out of the mountains on the east side of the Madison, Dr. Peale and Mr. 

 Holmes found that the strata were inverted. In the canon of Jackass 

 Creek all the beds, from the lowest Silurian to the Tertiar^^ inclusive, 

 were inverted so that the youngest Tertiary beds were at the bottom in 

 order of superposition. For a more complete account of the geology of 

 this region, as well as the Cherry Creek mines, the reader is referred to 

 the report of Dr. Peale. 



CHAPTER IV. 



MADISON ^VALLEY— THEEE FOEKS— GALLATIN VALLEY 

 AND CANON TO SOUECE OF ElVEE— FEOiAI THEEE FOEKS 

 TO HELENA. 



Just below the mouth of Elk Creek, the Madison Valley expands 

 into an open basin with high, rather rounded hills of the lake deposits 

 on the east side about ten miles distant from the rim, while on the 

 west side are bluff-lands, cut by the river, exposing the strata clearly 

 and showing their horizontal position. In none of the upper basins are 

 the lake-deposits as well exposed, and the character of the sediments 

 shows that they were deposited in comparatively quiet waters. The 

 long i)oint or tongue which extends down to the junction of the Forks, 

 between the Madison and the Gallatin, is composed of these deposits, 

 and in the ravines, which in some places cut deep into the ridge, large 

 masses of very beautiful silicified wood are found. I have no doubt 

 that bones might be found by diligent search ; for in the Jefferson 

 Valley, in the same kind of deposits, I discovered the teeth and jaws of 

 an Anchitherium, and a species of Helix. Of course these basins were 

 all connected at one time far up the valley of the Jefferson as well as 

 the Gallatin, but during the gradual period of subsidence became dis- 

 connected and ended in quite distinct lake-basins. The South Bowlder 

 Eange formed a shore-line on the west side. The waters must have 

 reached so high up on the sides that little more than the summits of 

 the peaks were above the surface and therefore most of the ranges were 

 then represented only by small islands. The Madison and the Gallatin 

 Eanges on the east were also shore-lines, but became more conspicuous 

 as the waters diminished ; and while thin patches of the peculiar lake- 

 5 G s 



