122 DESCEIPTIVE GEOLOGY. 



trachytes, which extend westward nearly to the West Fork of the North 

 Platte, where they end abruptly in steep cliffs. This trachytic area stretches 

 over 20 miles, in an east and west direction. 



In its lithological aspect, this area of intrusive rocks, lying between the 

 North and Middle Parks, presents several features of special interest, which 

 distinguish it from all others, and call for something more than mere men- 

 tion. Indeed, none of the trachytes show the same physical habit which 

 marks the outbursts of this species in Utah and Nevada. 



Just beyond the limits of the map, and to the east of Parkview Peak, 

 near the trail which goes over into Middle Park, occurs a number of isolated 

 hills and cones of a somewhat peculiar composition. That the rocks are 

 intrusive there can be no doubt, and at the time of their examination in the 

 field were regarded as belonging to the trachytic family, both from their pecu- 

 liar habit and general appearance, and from their mode of weathering, and 

 association with related rocks. At that time, however, no definite evidence 

 was obtained as to their age, as is the case with the eruptive rocks to the 

 westward. Subsequent detailed study of hand-specimens threw some doubt 

 upon the reference to the trachytes, and examinations of thin sections imder 

 the microscope led Professor Zirkel to place them among granite-porphyries. 

 The arguments brought forward in favor of this latter classification are so 

 strong that it would seem highly probably that they were older than any 

 trachytic eruption. On these grounds, we have been led to regard them 

 provisionally as granite-porphyries, possibly breaking through the Archsean 

 foundation, and occurring here as an outlier from the Medicine Bow Pange. 



With these rocks, Professor Zirkel has classed others from the summit 

 of Parkview Peak, which, in most of their microscopical details, develop 

 the same characteristics, but, in their field habit, present still more the aspect 

 of trachytes ; and, as to their age, it would seem quite clear that they have 

 broken through Cretaceous strata. 



The rock from the pass east of Parkview Peak possesses a decidedly 

 porphyritic texture, with large, well-defined crystals, chiefly feldspar, em- 

 bedded in a fine-grained, brownish-gray groundmass. It weathers in large 

 angular blocks, and, in its broader outlines, shows great diversity of form. 

 On the surface, it appears somewhat porous, owing to the decomposition of 



