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University of California. 



[Vol. 2. 



modified by secondary action. Besides the iron oxide, opal per- 

 meates the rock, filling cracks and considerable areas between the 

 crystals. The cementing material is quite obscured by these sec- 

 ondary products, but may, in part, be fine volcanic ash. It is in 

 this andesite breccia that analcite was found cementing the frag- 

 ments and replacing the feldspars as described on a previous page. 



The 20-foot lens of pale gray, finely laminated andesite which 

 has been described as occurring between the holocrystalline and 

 porphyritic lavas which make up the Grizzly Peak andesite belt, 

 is allied in character to the variety next to be described, but, on 

 account of its association, may be here referred to. In thin section 

 the rock is seen to have a very fine granular structure, with occa- 

 sional small phenocrysts of feldspar and augite. The ground-mass 

 is chiefly composed of feldspar microlites and granules, with a 

 slight admixture of augite and magnetite grains and little or no 

 glass. The laminated structure so prominent macroscopically is 

 evidenced in the slide by narrow zones traversing the rock at 

 small intervals, in which the feldspar microlites are parallel, the 

 intervening spaces showing no such grouping. The yellowish 

 tint of the separation planes is seen to be caused by the concen- 

 tration of a pale yellowish serpentine-like mineral along these 

 zones, which imparts its tint to the feldspar, and thus appears to the 

 eye to be crystalline-granular. 



Apliajiitic Laminated Andesite. — This designation is applied to 

 certain peculiar lavas which are prominently developed in the 

 Campan basin, particularly on Pie Knob and Fog Bluff. The type 

 is confined to the Campan series, in which they occur among the 

 earlier flows. The rock is of a light gray to reddish color, with a 

 well-defined lamination evidently due to flow. As a rule, it is 

 quite aphanitic, although porphyritic crystals may be occasionally 

 detected. Partings are not uncommonly developed along the 

 planes of lamination, but between these the rock is compact in 

 texture, although it may be locally vesicular. This rock seems 

 to have been affected more largely by processes of alteration than 

 any other rocks in the volcanic series. Large masses are seen 

 seamed and fractured in every direction, and intersected by slick- 

 ensided surfaces of greater or less extent. Secondary minerals 



