Vol. 4] Enopf-Thelen. — Geology of Mineral King. 



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show some rounding of the edges, and even embayment, or may 

 be surrounded by a resistant aureole which causes them to 

 weather in relief. Aplite dykes and pegmatites are common at 

 the contacts and vicinity, and were noted to fault from two or 

 three inches the basic segregations previously described. 



CONTACT METAMORPIIISM. 



The conditions are distinctly unfavorable for the systematic 

 study of contact metamorphism. This is principally due to the 

 fact that in the great majority of cases the strike of the strata 

 is nearly parallel to the contact. Both ends of the belt are unfor- 

 tunately obscured, either by morainal debris or by decomposition 

 products. Other deterrent factors are the shattered condition 

 of the strata near the granites, and the frequently inaccessible 

 position of the contact on precipitous cirque walls. 



This last cause was especially felt in the case of a meta- 

 morphic clay slate in the Crystal Lake cirque. The contact could 

 not be approached closer than about 400 feet. At this distance 

 the rock is dense and fine grained, massive, and of dark slate 

 blue color. Scattered through it in sparse porphyritic fashion 

 are small prisms of tourmaline, 1 mm. in diameter and several 

 mm. in length. Under the microscope the tourmaline shows 

 a nearly circular cross-section of yellowish brown color. The 

 section is fairly homogeneous and free from inclusions of the 

 groundmass, but is surrounded by a heavy peripheral fringe 

 of carbonaceous material. The tourmaline contains some liquid 

 inclusions which are disposed in radial fashion. The matrix is 

 composed of rectangular and quadratic andalusite sections em- 

 bedded in a small amount of interstitial quartz, and a large 

 quantity of black opaque material, forming nearly half the bulk 

 of the slide. Ignition caused the rock to bleach, proving that the 

 included material is of organic origin. The andalusite is filled 

 with carbonaceous matter, which, however, shows little or no 

 tendency to systematic orientation. In one or two instances 

 only was a crude chiastolite cross noted. Pleochroism is absent, 

 and differences of absorption are feeble. Some small patches of 

 brownish biotite are also found. 



