LINCOLN rORPHYRY. " 79 



on the other hand, is often so far develo2:)ecl that they are not readily dis- 

 tinguished by the untechnical eye from granites ; as such, indeed, they are 

 frequently classed by the miners. A careful examination, however, readily 

 reveals their structural difference, which is that in them the larger crystals 

 are inclosed in a finer-grained groundraass, whereas between the crystals of 

 granite there is no such intervening and apparently structureless material. 



The principal subdivision of this group has been called Lincoln 

 Porphyry, from the fact that it is typically developed in the mountain mass 

 around Mount Lincoln. Its most striking characteristic is the frequent 

 occurrence of large crystals of pinkish orthoclase, from one inch upwards in 

 size, with a peculiar luster like that of sanidine. Plagioclase is generally 

 in small, white, opaque crystals. Quartz occurs in double-pointed hexagonal 

 pyramids, which have a rounded outline on fracture surfaces and often a 

 slightly roseate tint. Mica is found in small hexagonal plates, generally 

 decomposed and of greenish color. The microscope discloses, in addition 

 to the above minerals, allanite, zircon, magnetite, titanite, and apatite. No 

 microfelsitic or glassy matter is found in any rock of this type and no glass 

 inclusions occur in the Mount Lincoln rock. Orthoclase feldspar predomi- 

 nates in the groundmass and in the rock as a whole, while among the 

 porphyritic crystals of rocks, in which the characteristic large orthoclase 

 are wanting, plagioclase is in relatively larger proportion. Owing to the 

 size of the crystals, large masses of the rock have at a little distance a 

 decidedly granitic appearance. On weathered surfaces, especially in the 

 dry region of the mountain peaks, it is of light-gray color, somewhat 

 bleached, and often slightly stained by hydrous oxide of iron. In mine 

 workings, on the other hand, when freshly broken it lias a decidedly greenish 

 tint, from the change of biotite into chlorite. 



Occurrence. — The main development of the typical Lincoln Porphyry is 

 in the neighborhood of Mount Lincoln, where it occupies the same position 

 with regard to the ore deposits of that region that the White Porphyry does 

 about Leadville. It forms the immediate summit of Mount Lincoln, where 

 it is apparently the remains of a laccolitic body or head of a channel of 

 eruption. It occurs as an interbedded sheet in the Cambrian and forms 

 several large bodies, apparently interbedded sheets, in the Weber Grits 



