40 GEOLOGICAl. SURVEY OF THE TERRITOEIES. 



ible at the surface or not. As a rule, they are separated iuto thin layers, 

 with a great variety of texture, from the most unyielding quirtzite to 

 rotten gneiss. There are also distinct intercalated layers of clay or sand. 

 As a rule, these rocks become more massive as we descend; the softer 

 beds of clay and sand cease, until we find nothing but massive beds, hun- 

 dreds of feet in thickness, of homogeneous granite. All these rocks have 

 suffered erosion to a greater or less degree — sometimes they are entirely 

 swept away, down to the massive granites. • It is in the series of meta- 

 morphic strata, estimated to be several thousand feet in thickness, that 

 the principal deposits of gold and silver, in the Territories of Montana 

 and Colorado, are found. The altitude of these rocks depends, of course, 

 on the forces that have operated in the past to elevate the ranges of 

 mountains. At any rate, there is no uniformity any more than there is 

 in the surface of the country at the present time. We know one thing, 

 however, that as a rule the oldest of these granite rocks crown the loftiest 

 of the mountain ranges. The relations which the well-marked, stratified 

 granites sustain to the older and more massive granites is nowhere 

 better shown than in the mining regions of Colorado, especially at 

 Central City and Georgetown. 



In general terms, we speak of the geological structure of Montana as 

 extremely simple ; and so it appears to be ; but when wrought out with 

 the care that will be absolutely necessary to a truthful delineation of 

 the details, it will be found to be exceedingly comi^licated. We maybe 

 examining one of the mining districts, for example, and we may con- 

 clude that only metamorphic strata will be found over the entire area 

 occupied by the mines ; but perhaps, on a careful study of the details, we 

 shall find everywhere scattered about patches of all the Paleozoic rocks 

 known in the West, and quite possibly portions of the Mezozoic and Ceuo- 

 zoic also. In the valleys and gulches, upon the summits of the highest 

 mountains, and in the most nnexpected i:)laces, fragments of the Carbon- 

 iferous limestones will be found. We may take the position therefore 

 that the entire surface of the country has been at one time covered with 

 a greater or less thickness of sedimentary rocks. It is possible, though not 

 at all probable, that there are restricted areas in this portion of the West 

 where no unchanged sedimentary deposits have ever existed, and it is pos- 

 sible that over considerable areas no strata newer than Carboniferous may 

 have been laid down. There is reason to believe, however, that the entire 

 series of strata known in the northwest, above the metamorphic rocks, 

 were originally deposited all over the Territory of Montana. We may 

 conclude, therefore, that the erosive forces have operated with great 

 power in the district around Virginia City, stripping bare to the meta- 

 morphic beds, large areas. In the mining districts, in connection with 

 these agencies, was the wearing out of so many gorges, or gulches, as they 

 are usually termed by the miners. We may take as an illustration some 

 rather prominent streams in the vicinity of Virginia City j and if a care- 

 ful detailed survey were made, we should find that there is a main val- 

 ley or gulch, with great numbers of side-gulches running up into the 

 heart of the mountains on either side. The main stream may be fifty to 

 one hundred miles in length, and on either side are these branch gulches, 

 usually from three to ten miles long. These gulches may be carved 

 entirely out of the massive strata, or they may be partly due to erosion, 

 sind partly to an interval, formed during elevation, that is, a monoclinal 

 valley. The influence of the erosive forces, which acted with great power, 

 and probably through long periods of time, though widely distributed, 

 are local in their results. In other words, while the erosive forces were 

 in operation all over the West, there was no widespread connection, 



