362 DESCRIPTIVE GEOLOGY. 



already mentioned at the Miller Mine. It is a dark-green compact rock, 

 having on its weathered surface a porphyritical appearance, as the crj^stals 

 of feldspar and hornblende show more distinctly thus than in the fresh 

 fractures. By the aid of the microscope, it is seen that the fresher horn- 

 blendes contain magnetite grains, and within the groundmass is a certain 

 amount of glassy base. This body, as do most of the intrusive dikes 

 through this region, lies parallel with the stratification, and occupies the 

 position of a band of metamorphic slates just above the Ute limestone at 

 Alta, which along the spur southwest of the town is seen to have a more 

 and more felsitic character as one approaches the divide. 



Paleozoic Rocks. — The Palaeozoic beds of this region, which fold 

 around and partly cover the granite body, have been subjected to intense 

 compression and local metamprphism, twisted and contorted in every direc- 

 tion, faulted and dislocated, and penetrated by intrusive dikes and mineral 

 veins. It does not come within the scope of this work to delineate these 

 phenomena in all their details, but the main features of their structure have 

 been determined with sufficient accuracy for purposes of general geology. 

 The great belt of Wahsatch limestone, which forms the main stratigraphical 

 landmark in these formations, crosses the head of Little Cottonwood Canon 

 in a northwest and southeast direction, having an average dip of a.bout 45° 

 to the northeast, and bending, as has been seen, sharply to the southwest at 

 -the head of American Fork. It forms massive cliffs at the southern head 

 of Little Cottonwood, and here already, in the mantling of white, through 

 its general blue color, shows the commencement of the metamorphism which 

 has marbleized its beds in great degree from here to the mouth of Mill 

 Creek Canon, where they disappear beneath the Salt Lake plain. This belt 

 includes, as has already been stated, the three groups of Lower Coal-Meas- 

 ures, Sub-Carboniferous, and Nevada Devonian. 



On the ridge which separates the head of Little from that of Big Cotton- 

 wood Canon, these limestones have been eroded off, and the underlying 

 granite exposed. On the west face of this ridge, however, just above and to 

 the east of the town of Alta, a fragment of white limestone beds is still stand- 

 ing, while at the foot of the ridge the granite is again denuded in the cation- 

 bottom, showing that this depression, which forms the basin-like head of the 



