CHAPTER VI. 



yolcanio eocks.— geneeal chaeactee and distei- 



butio:n^. 



That series of rock formations whicli owes its origin to volcanic or 

 other igneous action is botli varied and extensively distributed over the 

 district. As here referred to, it embraces none biit the later eruptive 

 materials, since it is by no means certain that the granites in this region 

 are of the same origin. Yet in the Teton Mountains, where some of the 

 most lofty heights are blocked out of granite, intrusions of trap-like 

 material are found in nearly vertical fractures which separated the 

 inclosing rocks along lines more or less transverse to the axis of up- 

 heaval. Such trapean dikes are conspicuously displayed on the south 

 shoulder of Mount Hayden and in the huge bulk of Mount Moran. 

 Yein formations traverse the Archaean rocks wherever they are exposed 

 to view. 



Eruptive and intrusive volcanics. — Of the purely eruptive and intrusive 

 volcanics of later origin, we have fine examples in the gray hornblendic 

 trachytes of Mount Bainbridge in the Caribou Eange, and the little dike- 

 ridge at Station XVII in WiUow Creek Basin, a few miles to the north- 

 west. No data were observed by which the age of the latter deposits 

 might be fixed with accuracy ; but hand-specimens that were brought 

 from both these localities were recognized by Dr. Peale as having inti- 

 mate relationship, in both their hthology and mineralogical constitu- 

 ents, with the hornblendic trachytes of the Elk Mountains of Colorado. 

 These Mount Bainbridge intrusions are, however, very variable in their 

 physical constitution, often masses of the rock assuming a schistose 

 character suggesting their possible origin as fragments of the crystal- 

 line schists, which were entangled in the molten matter in its passage 

 through the metamorphic horizons ; but this would hardly seem to 

 explain their appearance in the intrusive sheets separating the sedi- 

 mentary beds, which often show this structure throughout, and which 

 may in some way be due to the presence of mica as one of the compo- 

 nents of the rock. The local and very variable aspects of the dike-like 

 and intrusive masses in Mount Bainbridge and at Station XYII have 

 already been noticed, as also the association, at the former locality, of 

 auriferous lodes from which the placer-gold mined in that neighborhood 

 was derived. We have, therefore, in these two observed locahties^ 

 eruptive mountain masses precisely similar to the isolated mountain 

 clusters that protrude in the great sedimentary plateau of the Colorado- 

 region of Western Colorado and contiguous regions of the neighboring 

 Territories, which have been so well investigated in the progTess of the 

 survey in that quarter. The trend of the dike-like mass in Station 

 XYII, which, extended southeast, almost exactly intersects Mount Bain- 

 bridge, strongly suggests the probable intimate relations of the at pres- 

 ent isolated masses — the intervening basin plain being covered by the 

 later flows of basaltic lava — though the eruptive matter may not have 

 protruded to a unifoxm height over this extent, but it seems to have 

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