GEANITE AND AECH^AN EOCKS. 361 



in little filaments and veins the crystals of quartz and feldspar, thus proving 

 it to be a later impregnation. 



In the Cambrian glates of 'Twin Peak, on its northwestern slope, and 

 partly interstratified, is an interesting porphyry, which, in the hand-speci- 

 men, has much the feel and general appearance of a trachyte. It is a 

 reddish-drab colored rock, consisting of a rough-feeling, rather granular, fine- 

 grained groundmass, in which is imbedded an immense quantity of bronze- 

 colored mica plates. By the naked eye, no other crystals can be distin- 

 guished; but, under the microscope, the groundmass is seen to be made up 

 of crystals of a dull feldspar, the intermediate spaces being filled with quartz, 

 small brown micas, and apatites, but no hornblende or titanite. The same 

 rock, with perhaps even a larger proportion of mica crystals, is found cutting 

 through the granite spur near the mouth of Big Cottonwood Canon. 



Another type of granite-porphyry is found north of the granite body 

 of Clayton's Peak, on the divide between the headwaters of East Canon 

 Creek and of Big Cottonwood. It has a pale-green groundmass, containing 

 large crystals of pinkish- white feldspar and prismatic columns of dull-green 

 hornblende porphyritically enclosed, while the whole mass is impregnated 

 with pyrites. The hornblende crystals have frequently a radial grouping. 

 The feldspar resembles, at first sight, orthoclase, but is seen to have the 

 characteristic striations of plagioclase. Under the microscope, the color of 

 these larger feldspars is seen to be due to a fine dust of oxide of iron, and 

 that of the groundmass to epidote, resulting from the alteration of horn- 

 blende, while its feldspars are largely plagioclase. 



Still another granite-porphyry is found in the Weber Quartzites east of 

 Clayton's Peak, near their contact with the trachyte body at the head of 

 Provo Valley. This is a gray granitoid rock, made up of quartz, feldspar, 

 biotite, and hornblende, in grayish crystalhne groundmass. The horn- 

 blendes are in long prisms of a dull-green color, and seem to be generally 

 arranged in parallel directions. Here also, as in the granites, plagioclase is 

 a prominent constituent. 



On the divide between the southern head of Little Cottonwood Canon 

 and that of American Fork is a body of diorite, which occurs near the 

 base of the Ogden Qiiartzite, and seems to be on a line with the break 



