306 EXPLORATIONS ACROSS THE GREAT BASIN OF UTAH. 



No. 132. From the hills west of Kamas Prairie. Is a dull gray, finely vesicular 

 rock. It shows a great tendency to crystallization, containing numerous minute, indis- 

 tinct crystals, of a blackish-green mineral, probably hornblende (not olivine), and some 

 lamina' of brown mica, It may be a vesicular form of the rock No. 152, and belong 

 to the same group, although its la vatic appearance seems to point to a more modern 



Rocks similar to one or the other of this series have been found in various local- 

 ities — near Simpson's Spring (No. 178); in the McDowell Mountains (No. 385); in Butte 

 Valley (No. 240); and especially in the western part of this section, in the Se-day-e 

 Mountains (Nos. 293, 295, and 341), and near Carson River (Nos. 318, 320, 333, 334, 

 and others). They are all composed of a feldspathic matrix, with crystals of feldspar, 

 and either hornblende alone, or hornblende and mica, quartz and mica, or hornblende, 

 quartz, and mica, subject to the same law of mutual substitution. Still I am not 

 certain if all these rocks belong to the same group. The feldspar in some of them 

 may be the glassy feldspar, sanidine, which is characteristic of the trachytic group. 

 They would then have to be called trachytes, trachytic porphyries, and trachytic 



The extreme type of another class of rocks is to be found in No. 181 of the col- 

 lection, a porphyritic ruck from Simpson's Spring, on the eastern rim of the Great 

 Salt Lake Desert. In its compact matrix light pink and white are mixed It contains 

 numerous crystals <>t mostly dark-colored quartz, and, somewhat less prominent, but 

 also in large quantity, crystals of light greenish feldspar, orthoclase, with highly per- 

 fect cleavage in three or four directions, and, with difficulty, fusible at the edges, 

 before the blowpipe. I also notice many small scales of dark green mica. In some 

 portions of the rock pink prevails, and in others a light greenish-yellow, without any 

 red; but in all the varieties the crystals of quartz are most prominent. This porphyry, 

 if observed alone, might readily be considered as one of the old porphyries, allied and 

 coeval with the granitic group; but I find in the collection a series of specimens which 

 show that it is allied to rocks of a much more modern appearance, and prove, beyond 

 doubt, its close connection with the trachytic porphyries. The feldspar of the other 

 rocks belonging to that group exhibits a more glassy fracture, and a cleavage which is 

 not so perfect in all directions. Most similar to it is the porphyry from Good Indian 

 Spring, in the McDowell Mountains (No. 382), and specimens from Eagle Valley and 

 Carson River, near the Sierra Nevada (Nos. 326, 316, &c). Allied rocks were fre- 

 quently met with. The quartz in many of them is highly brittle and perfectly crys- 

 tallized in hexagonal double pyramids. Others show a certain want of cohesion, which 

 is uncommon with older rocks. This group was found merging, by intermediate 

 forms, as well into the preceding series, as also into others described below, so much 

 so that the position of single specimens becomes doubtful. 



Another extreme type is represented by specimen No. 222, from the Ungo-we-ah 

 range. It is a porphyry, with a fine, chocolate-colored matrix and even "fracture, 

 inclosing numerous small crystallizations of white feldspar, besides which only minute' 

 black particles, probably of hornblende, could be distinguished. 



Similar rocks have been found largely developed in many of the mountain-ranges. 



