98 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



tlirougliout the mass. The associated gneiss also contains large ort^aoclase 

 crystals, often two inches in length and usually Carlsbad twins. On the 

 surface of this floor was observed an interrupted dike of hornblende-i)or- 

 phyrite, which is figured on the map ; also, small outcrops of other eruptive 

 rocks, notably one of White Porphyry, whose outlines Avere not determined 

 with sufficient accuracy to be there indicated. On the west wall of this 

 amphitheater appears a dark line, which may probably be part of the same 

 dike of jwrphyrite as is shown on the map to extend almost continuously 

 along the west wall of the Arkansas amphitheater. Owing to their darker 

 color and peculiar fracture in large masses, which is like that of a basalt or 

 andesite, the porphyrite bodies can readily be distinguished at a consider- 

 able distance. 



The North Peak ridge, which forms the northern wall of the Platte 

 gorge, being lower than the corresponding spurs to the north and south, 

 respectively, is composed almost entirely of Archean rocks, a proportion- 

 ately smaller capping of Paleozoic strata being left on its crest. The actual 

 outline of the remnant of Cambrian quartzite remaining on the ridge could 

 ordy be determined with exactness by the expenditure of more time than it 

 was possible ty devote to this point, and the line given on the map is that 

 determined by observation of the apparent stratification, lines from Mount 

 Lincoln. 



Quandary Peak. — On the Quaudary Peak ridge, which lies just north of 

 the limits of the map, it is easily seen from a distance that a remnant of 

 Lower Quartzite is left at the very summit of the peak, as shown in the 

 sketch given in Plate X, which is taken from the summit of Mount Lincoln. 

 The angle of inclination of these beds, which is 15°, is less than that of a por- 

 tion of tlie ridge, in consequence of which they have been eroded off the saddle 

 immediately east of the peak, and are found again lower down on the spur. 

 At the timber-line, which reaches only the eastern end of this spur, the dip 

 steepens to 25°. This line of steepened dip can be traced on all the prin- 

 cipal eastern spurs of the range and corresponds ver)^ nearly with the mouth 

 of the canon gorges which have been cut in the Archean. It is often accom- 

 panied by some apparent dislocation of the strata, the amount of which, 

 owing to discordant dip angles, it was not easy to determine. For pros- 



