HAYDEx.] GEOLOGY OF THE ELK MOUNTAINS. 55 



range, in the South Park range, the eruptive groups have thrown the 

 sedimentary beds into the utmost confusion, producing those remarkable 

 faults and irregularities which were shown in the annual report for 

 1873. On the west side, in the Elk Mountains, the confusion is still 

 greater, producing not only the most remarkable faults in all the western 

 country, but literally overturning thousands of feet of strata. By exam- 

 ining the preliminary map of Colorado in this report, it will be seen that 

 the trend of the iSawatch range is very nearly north and south, and tbafc 

 the principal peaks from the Mountain of the Holy Cross at the north to 

 Mount Ouray to the south lie along that line. The trend of the Elk group, 

 though less regular, will be seen to be about northwest and south- 

 east. This is a grand illustration of an eruptive range, and appears 

 also to be an example of a sudden violent or catastrophic action. The im- 

 mense faults, complete overturning of thousands of feet of strata, and the 

 great number of peaks, all composed of eruptive rocks, indicate, perhaps, 

 periodical and violent action in contradistinction to Ion g-con tinned uniform 

 movements of the elevatory forces. The sections and maps which accom- 

 pany this report will doubtless enable the geologist to determine the cor- 

 rectness of our statements. The map will show by the colors the erup- 

 tive points, where the granite appears to have been thrust up, as it were, 

 through the vast overlying crust; sometimes a great thickness of 

 strata of various ages is carried up to the summits of the peaks, 13,000 

 or 14,000 feet in elevation above the sea. Again we find, but a few 

 yards away, the same group of strata in the bottom of the lowest val- 

 leys, indicating remarkable convulsive movements. Although the Elk 

 group may be regarded as an eruptive range, it will be seen by the map 

 and section that the elevatory forces, whether convulsive or uniform 

 and slow, acted along a well-defined axis, thus, as a range, forming a true 

 anticlinal. 



We see, therefore, that the eruptive agents acted along a great fissure 

 in the earth's crust as a line of greatest weakness, and that this line 

 possessed a trend about northwest and southeast. But the peculiar nature 

 of the forces produced the wonderful chaos in the position of the sedi- 

 mentary beds, while the tendency of these strata is toincline from either side 

 of the axis. It is not uncommon to find thousands of feet of strata which 

 have been carried up to the loftiest points of the axial ridge in nearly or 

 quite a horizontal position. We may suppose that at one period the vast 

 sedimentary mass rested on a floor of pasty or semi-pasty granite; that 

 the forces in the interior were struggling to find vent, carried upward 

 the entire overlying mass of sedimentary strata, and that here and 

 there many thousands of feet in thickness along the axial line or ridge 

 was thrust up through the melted or serai-melted granite in such 

 masses as are shown on the map, at Italia, White Eock, Snow Mass, Capi- 

 tol and Sopris Peaks. The map will show that this igneous granite does 

 not reveal itself except along this quite regular axial line. The areas 

 of granite are greatly enlarged, by subsequent erosive action, while 

 from the axis numerous streams cut deep gorges, 1,500 to 3,000 feet in 

 depth, sometimes far into the underlying floor of igneous granite. 

 During this period of revolution, and probably subsequently, there were 

 igreat numbers of dykes or orifices from which issued the rhyolites and 

 fbasalts. Gothic and Crested peaks are illustrations of the upthrust of 

 vast masses of rhyolite, and numerous other quite long dykes will be 

 noticed on the map. " 



Plate XIV represents a portion of the east face of Gothic Mountain, 

 the central mass of which is rhyolite, with only the Cretaceous beds 

 lilted up around, the base and sides. This is an excellent example of these 



