MOUNT LINCOLN. 113 



with some difficulty, which shows the characteristic large, pink, orthoclase 

 feldspars of the Lincoln Porphyry. In this the green color is seen to be 

 due to the alteration of biotite into a chloritic substance, which has been 

 deposited on the surface of the smaller feldspars, so that they are scarcely 

 distinguishable by the naked eye. Biotite is also no longer visible except 

 under the "microscope. Pyrite can be distinguished throughout the rock by 

 the naked eye. 



The Archean rocks (a) at the base of this section consist almost en- 

 tirely of dark-gray gneiss. In this the White Porphyry dike can be traced 

 but a short distance, as it is soon lost under the steep talus slopes at the 

 foot of the cliff. 



A few hundred feet east of this dike (to the right in the sketch) a second 

 dike (No 2) can be traced, though less distinctly, from the gneiss entirely 

 across the Cambrian and Silurian formations, apparently terminating at the 

 base of the Blue Limestone. It is much narrower and straighter than dike 

 No. 1, and like that seems to have a northeast and southwest direction. Its 

 rock is a light-colored, fine-grained, highly-crystalline porphyry, belonging 

 to the type designated as Mosquito Porphyry, which has already been de- 

 scribed. There is an outcrop of the sama rock in the Archean, on the west 

 wall of the Cameron amphitheater directly under Mount Cameron, which 

 may possibly be part of the same body, although the intermediate region 

 is too much obscured by de'bris to trace any direct connection. 



Eastward of this cliff face the northern wall of the Cameron amphi- 

 theater is much covered by de'bris for the distance of nearly a mile, in which 

 extent, although the general dip of the sedimentary beds can be traced, no 

 opportunity was presented for an examination of the intrusive bodies. 

 Near the eastern end of the wall, however, is a second cliff section, which 

 shows in a very instructive manner the position of the intrusive masses and 

 dikes. It is graphically represented in Plate XII, which, like the preceding 

 plate, is copied from sketches made on the spot by Prof. A. Lakes. The 

 section was studied by Mr. Cross, from whose notes the following descrip- 

 tion is largely taken. 



Here, as in the section just described, is an intrusive interbedded mass 

 of porphyry in the Lower Quartzite (), only a few feet above its base, 



MON XII 8 



