188 GEOLOGICAL SUEVEY OF THE TERRITORIES. 



auiorphie ranges of mountains is northwest and southeast, and that the 

 eruptive trend northeast and southwest. The dikes that sometimes 

 extend long distances across the i^lains, in all cases trend northeast and 

 southwest, or occasionally east and west. The purely eruptive ranges 

 of the northern portion of the San Luis Valley seem to bo composed 

 of a series of minor ranges " en echelon^'' with a trend northeast and 

 I southwest. But as soon as this range joins on to a range v/ith a meta- 

 morphic or granitic nucleus, the trend changes around to northwest and 

 southeast. Many of the ranges have a nucleus of metamorphic rocks 

 though the central and highest jjortions may be composed of eruptive 

 peaks and ridges. In this case the igneous material is thrust up in lines 

 of the same direction as the trend. It becomes therefore evident that 

 all the operations of the eruptive forces were an event subsequent to 

 the elevation of the metamorphic nucleus. This is shown in hundreds 

 of instances in Southern Colorado and New Mexico, where the eruptive 

 material is oftentimes forced out over the metamorphic rocks, conceal- 

 ing them over large areas. 



All over the mining districts are well-marked anticlinal, synclinal, and 

 what I have called monoclinal valleys. ISTea^rly all the little streams flow 

 a, portion or all their way through these monoclinal valleys or rifts. In 

 most cases the streams pass along these rifts from source to mouth, but 

 occasionally burst through the upheaved ridges at right angles, and 

 resuming its course again in some monoclinal opening. There are a few 

 instances of these streams flowing along anticlinal valleys, and by anyone 

 these remarlvs will be at once understood by studying the myriad little 

 branches of Clear Creek or South Platte, which liow for long distances 

 through the mining districts. 



In these valleys are oftentimes accumulated immense deposits of 

 modern drift. Sometimes there are proofs' that tliese valleys have been 

 gorged for a time, and a bed of very coarse gravel and boulders will ac- 

 cum ulate, hundreds of feet in thickness. Near Georgetown there is a fine 

 exami^le of this modern drift action. 



It vvould seem that the valley of that branch of Clear Creek, in which 

 the Brown and Terrible silver lodes are located, was gorged at one time, 

 perhaps, with masses of ice, and the fine sand and coarse materials 

 accumulated against tlie gorge, and at a subsequent period the creek 

 wore a new channel through this material. The upper side of this drift 

 deposit is "fine sand, but the materials grow coarser as we descend, until, 

 at the lower side, there are immense irregular or partially worn masses 

 of granite. On the sides of the valley' the rocks are often much smoothed 

 and grooved as if hy floating masses of ice. We assume the position, of 

 which there is most ample evidence all over the Eocky Mountiiin region, 

 that at a comparatively modern geological period the temperature was 

 very much lower than at present, admitting of the accumuhition of vast 

 bodies of ice on the summits of the mountains. The valley of the South 

 Platte, as that stream flovv^s through the range east of the South Park, 

 show, not only these accumulations of very coarse boulder drift, but when 

 this drift is stripped off, the underlying rocks are found smoothed, and, 

 in some instances, scratched, as if by floating icebergs. 

 , In regard to the character of the gold and silver mines of Colorado, 

 much information of i)ractical value has been secured, but my limited 

 time will not i^ermit me to present it in detail in this preliminary report. 



It will be more fully elaborated during the coming winter. 1 would 

 simply remark that my observations indicate to me that the silver mines 

 of Georgetown are very rich and practically inexhaustible, and that, 

 under the present system of working them, they are l^ecomiug daily 



