382 GEOLOGY OF THE EUREKA DISTRICT. 



crystalline, the outline of the brecciated fragments being sharp and well defined. 

 Thin section 189, taken from a spot 18 inches distant from the line of contact, shows 

 the effects of partial remelting, the character and composition of the breccia being the 

 same as in the last thin section. The greatest change is noticed in the colorless 

 porous fragments, where the size of the fluid and gas cavities has been greatly 

 reduced, the whole seeming to be contracted and crumpled together; there begin to 

 appear also in the place of the cavities minute black grains and microlites in small 

 numbers. The definition of the pumice fragments is no longer marked, and they 

 commence to merge in the surrounding matrix. 



In thin sections 193, 191, and 192, from immediate contact with the basalt, where 

 the fusion has been complete, the resulting body is a compact glass almost free from 

 gas or fluid inclusions, which have been driven out by the heat, since the mass was 

 under little or no pressure. The glass in some instances, as in section 192, has retained 

 its former brecciated character, preserving the outline of its component fragments, but 

 has so contracted as to present many more phenocrysts to the same area of thin section 

 and has become of very dark, blue-black color. This color seems to be due to innumer- 

 able black hair-like trichites, opaque grains, and a smaller number of transparent 

 microlites, both short and stout and long and curved. There are in this thin section 

 portions of the neighboring basalt having an exceptionally dark brown glass base. In 

 193 and 191 the evidence of a former brecciation has almost entirely vanished ; the glass 

 of the different fragments in- some places has been very uniformly mingled, especially 

 in 193, though occasional fragments have offered greater resistance to fusion. The 

 lighter color of 193 is due to the reflection of light from mist-like clouds of gas bubbles 

 of the minutest dimensions, which appear at first to be opaque particles, but are found 

 under a power of 850 diameters to be transparent globules, with a heavy dark border. 

 They are especially abundant around two small cavities in thin section 191 and prob- 

 ably cause the yellowish white lining of the larger cavities in the hand specimen, 

 which is peculiar to several occurrences. Thin section 196 is from another form of 

 alteration of the same pumice ; it is rather more crystalline and is filled with opaque 

 particles that are red and yellow in incident light and give the rock its color; it is 

 also very porous. 



The same effects have been produced in the pumice by the numerous outbreaks 

 of basalt along the south slope of Richmond Mountain, and the thin sections from 

 this locality present in many instances the same characters as those just described. 

 They will therefore need but a brief mention and will serve rather as evidence of the 

 identity of the two bodies of pumice and of the uniformity of the alteration arising 

 from the same cause. Thin sections 199, 200, 204, 205, 206, 207, 208, and 209 are from 

 rocks on the small spur south of the summit of the mountain, and occurring under 

 different conditions they vary somewhat in character. Thin section 19!) is of a fine 

 grained altered pumice not in immediate contact with basalt, and resembles thin 



