22 SURVEY OF COLORADO AND NEAV MEXICO. 



a moiiocliiial rift, or between two ridges, whether of changed or nnch.anged 

 rocks which incline in the same direction. We ascend to the axis 

 of the main Rocky Mountain range by a series of step-like ridges, each one 

 inclining in the same general direction at some angle, with their counter- 

 parts on the opposite side of the main axis. Speaking of these ridges 

 locally. I have called them in this report monoclinal, from the fact, that 

 as a rule their counterparts, although they have once existed on the west 

 side of the range, are in most cases swept away. We passed uj) a beau- 

 tiful valley Avith the red beds on our left, and a few remnants of the red 

 beds and the nietamorphic rocks on our right, for about fifteen miles. We 

 then came to the red feldspathic granites, in which the mineral lodes are 

 located. We first examined a local vein of black rock, in which horn- 

 blende predominates. It contains some mica and iron, so that it might 

 be called a local outcrop of black hornblende syenite. Masses of it 

 have a rusty look from the decomposition of the iron in the rock, and 

 sometimes it is covered with an incrustation of common salt or potash. 

 Iron in some form is one of the prominent constituents of all the rocks 

 of this region, changed or unchanged. So far as I could determine, the 

 inclination of the metamorphic rocks is in the same direction as the sed- 

 imentary. 1 have assumed the position that all the rocks of the West 

 are, or were, stratified, and that where no lines of stratification can be 

 seen, as in some oi" the massive granites, they have been obliterated by 

 heat during their metamorphism. Therefore all the metamorphic rocks, 

 whether stratified or massive, that form tlie nucleus of the Rocky Moun- 

 tains, nnist have some angle of dip, e(jually with the sedimentary rocks. 

 In many cases I have to be guided by the intercalated beds of mica or 

 talcose slates. I am of the o}>inion that there are anticlinals and syn- 

 clinals among the metaniorphic rocks of this region, and that the mount- 

 ain valleys are thus formed for tlie most part. 



We examined a number of lodes which were moderately rich in copper. 

 All the lodes have a trend about northeast and southwest, and are two 

 to four feet wide, with well-defined walls. Much of the gaugue rock is 

 spongy like slag, owing to the decomposition of iron pyrites ; and there 

 are large masses of the casts of cubes, evidently cubes of iron pyrites. 

 Our examinations were not verj^ thorough, but I was not favorably im- 

 pressed with this district as a rich mineral region. Some of the copper 

 mines, at some future day, may yield a fair retuin, but it will be many 

 years before the country will be built up by its mineral wealth. 



July 3. — Oar route to-day was along the flanks of the mountains, from 

 Cache a la Poudre to Big Thompson Creek. Ijying over the red beds 

 and appearing to form a dividing line between the red beds and the 

 ashen-graj' marly clays above, is a M^ell-detined bed of Iduisli semi-crys- 

 talliiie limestone, two to four feet thick, somewhat cherty, though sus- 

 ceptible of a high polish, too brittle and liable to fracture iu any 

 direction to be valuable for ornamental purposes — probably useful for 

 liine only. I regard this as of Jurassic age, although I was unable to 

 find in it any well-marked organic remains. The same bed occurs in the 

 Laramie plains, where it contains man^' fragments of crinoidal stems, 

 which Professor Agassiz referred to the well-known Jurassic genus 

 Apiocrinites. 



On the summit of the first main "hogback" is a bed of massive 

 sandstone, immense blocks of which have fallen down on the inner side 

 of the ridge, adding much to the wilduess as well as ruggedness of the 

 scenery. These rocks are made up almost entirely of an aggregation of 

 , small water- worn pebbles. The layers of deposition are very irregular, 

 iiucliuiug at various angles. This irregularity in the laminoe is a nuirked 



