304 EXPLORATIONS ACROSS THE GREAT BASIN OF UTAH. 



the Pe-er-re-ah range, longitude 116° 50', and latitude 39° 30'; in the Se-day-e 

 Mountains, longitude 117° 30', and latitude 39° 13'; and in the Sierra Nevada, in the 

 Carson River Canon, longitude 120°, and latitude 39°. Rocks of granitic (?) appear- 

 ance, but of doubtful character, occur also in the Mon-tim range, longitude 115°, lati- 

 tude 39° 50'. 



Mr. Marcou, in his report on Captain Whipple's route, near parallel 35°, although 

 not entering much into the subject, makes distinctions in regard to the age of the 

 granites of different ranges, and I, too, am inclined to consider some of the granitic 

 rocks of this section as much more recent than the normal granites.* My specimens 

 trnin the Sierra Nevada contain a good deal of green hornblende, besides the mica, 

 which gives them a character not met with, to my knowledge, in the true granites of 

 the eastern hemisphere. Similar granites have been observed by Mr. Blake, near 

 Fort Miller, in Southern California, 



The granitic rock from the Wahsatch range, east-northeast of Camp Floyd, is 

 composed of albite r (?), quartz, and green mica, and appears to me much nearer allied to 

 the rocks of the dioritic group, which are of more recent age. 



All the other igneous rocks of the section are found merging into each other. I 

 shall confine myself to describe some of the most characteristic ones, and point out 

 their relation to others. 



In the Wahsatch range, between the Weber and Timpanogos Rivers, we find a 

 very instructive series of rocks, some of which have the appearance of normal trachyte, 

 or seem to be allied to the andesite, while they probably are porphyritic diorites.f 



Prof. Gustavus Rose remarks "that one might be frequently induced to group 

 the dioritic porphyries of this continent together with the andesite which belong to the 

 trachyric group, and is generally more recent than the diorites" ; and "that "the age 

 and general development of the American dioritic rocks does not seem to differ much 

 from that ot the trachytes, while in other countries they approach more the granitic 

 group/' 



My observations seem to confirm this remark, and I might be inclined to consider 

 tins senes of rocks as trachytic, if the feldspar, which they contain, although similar 

 in appearance to some varieties of the glassy feldspar, did not differ from it in the 

 degree of fusibility. An analysis would be required to determine its mineralou-ical 

 position. 



I will give a description of the most characteristic specimens of this series: 

 v N ° ™ * ° f f e C ° lleCti0n ' from the summit between Silver Creek and Timpanogos 

 Riyer. This rock may be regarded as the most normal of these porphvritic diorites 

 It has a dark-gray, granular, highly qn^zoBemafai^ hich, under the micros copes 



,..,-: ..,-,-- >: ..... ■"'.',."" 



utz. They form one group with the syenites, and certain anions ,„„ nhvri,; 

 mgerthau the Upper CarbooiferoM. 



They are also e 



i quartz. The n 



J frequently contains mica besid 



