PLATTE AMPHITHEATER. 97 



coin and at the head of the Cameron amphitheater. It is a Hght-gray, tine- 

 grained rock, consisting of quartz, feldspar, and biotite crystals in a very 

 scanty groundmass. The groundmass is a fine-grained mosaic of quartz, 

 with some feldspar and muscovite, the latter resulting from decomposition 

 of feldspar and probably in part also from fine biotite leaves, since this al- 

 teration is visible in the larger individuals. 



4. Just west of this is a very irregular body of quartz-porphyry, not 

 shown on the map. It occurs at the base of the clifi"s and is Very variable 

 in form and thickness, branching out irregulai'ly and continually changing 

 its direction. It is a dull-green rock and belongs to the Green Porphyry 

 type. It is rich in feldspar, with a few grains of quartz, and what is prob- 

 ably a decomposition product of hornblende which gives the color to the 

 rock. On the cleavage-planes are coatings of ejDidote. 



5. Still further up the valley, directly under the summit of Mount Lin- 

 coln, is a dike of White Porphyry extending high up on the face of the 

 cliff. It resembles closely the typical White or Leadville Porphyry, but is 

 less decomposed. A few small crystals of quartz and feldspar are visible, 

 also frequent light-green specks of partly decomposed biotite. It is almost 

 identical with the similarly situated dike (dike No. 1) in Cameron amphi- 

 theater, on the south face of Mount Lincoln, and with fragments found at 

 the head of Buckskin amphitheater, for which this description will also 

 apply. Its outer weathered surface is very white and homogeneous-looking; 

 immediately under this is a dark zone, less than an inch in thickness, which 

 apparently owes its color to the oxidation of some heavy metal originally 

 contained in ore particles or in the biotite. It was impossible to obtain 

 sufiicient biotite for a chemical test to prove this assumption, which is 

 founded on indications observed by the microscope. In the Cameron rock 

 small crystals of pyrite could be detected, and in that from Buckskin a 

 little galena also, whose decomposition would more directly account for the 

 dirty-brown color alluded to. 



On the raised floor of the northwestern arm of the Platte amphitheater 

 granite predominates among the Archean rocks. It is of the same variety 

 as that found directly west in Bartlett Mountain and Clinton amphitheater, 

 and has large and prominent crystals of feldspar disposed in regular order 



