Cambrian 



BED AMPHITHEATER 121 



\ Dark-green serpentine ......................... 10 



White limpid quartz ......................... 15 



Yellowish -greeu serpentine .................... 1 



Green porphyry, 4 feet. 



White quartzite ............................. 10 



Grecn 1'orphyry, 20 feet. 



Wliite quartzite ...................... . ........ 10 



Green porphyry, 5 feet. 



White qnartzite ............................... 40 



Intrusive mass, disturbing strata and disappearing 

 under debris ................ ............... (?) 



Neither the top of the Blue Limestone nor the base of the Cambrian is 

 reached in this section, and to the aggregate thickness given an unknown 

 amount, probably in the neighborhood of 200 feet, should be added. At 

 the head of the amphitheater above the Blue Limestone is a very thick body 

 of Lincoln Porphyry, above which, on the summit of the ridge and sepa- 

 rated by a low saddle from the main summit of Mount Bross, are intensely 

 altered shales, frequently chloritic, belonging to the Weber Shale formation. 



The development of serpentine, which elsewhere seems confined to the 

 "sandy limestones" of the upper part of the Cambrian, here extends, though 

 on a minor scale of development, a short distance into the silicious beds 

 below and up as far as the base of the Blue Limestone. The serpentines 

 obtained from here are remarkably beautiful rocks, grading in color from a 

 homogeneous yellow to a dark green, mixed with gray and having the 

 general effect of a veined verd-antique, although more critical examination 

 shows that the green and gray or yellow are a simple shading oft' and inter- 

 growth. In some cases thin, vein-like sheets seem to cross the strata, though 

 in general the development of serpentinous material is parallel to the strat- 

 ification. Under the microscope they are seen to contain a very consider- 

 able amount of calcite, an appearance which is confirmed by chemical 

 analysis. The development of serpentine is apparent, in looking at the 

 cliffs from a little distance, as a lenticular-shaped body, giving at first the 

 impression that it causes an actual thickening of the beds; but the measure- 

 ments given by the above section show that this is not the case, and the 

 chemical examination, which is discussed in Chapter VI, shows that this 



