No. 2318. PETROGRAPHY OF DIKE ROCKS OP IDAHO— SHANNON. 485 



are commonly merely spongy shells inclosing a large amount of the 

 material of very low refraction and double refraction which may in- 

 clude opal, tridymite, and zeolites. The augites which are near diop- 

 side in composition, are commonly colorless but frequently have a nar- 

 row, faintly pleochroic border of a pale green color which shows some 

 admixture of the aegirite molecule. There are clearly two generations 

 of the colored constituents of the rock. The apatites, which are 

 often of large size, are late in crystallization and often contain dust- 

 like inclusions, especially along the vertical axis. The iron ore 

 occurs in abundant small euhedral grains in the groundmass. The 

 flow structure of the rock is A^ery marked. The groundmass is mostly 

 colorless to brownish glass but in places it acts feebly on polarized 

 light. The rock from the border of the dike has smaller biotites and 

 the augites are even more spongy than in the main mass of the dike 

 and the groundmass is more completely glassy. This glassy ground- 

 mass contains much dust-like material which by its dispersion gives 

 the glass a brownish color suggesting kaolin. 



Since classifications are based upon tlie character of the feldspar, 

 and there is no feldspar present in uncrystallized base of this rock, 

 it is difficult to say whether the rock should be classed with the 

 minettes or the kersantites. The aegiritic rims of the augites and 

 the faintly double-refracting patches in the groundmass give the 

 rock an alkalic appearance in thin sections and it is placed with 

 the majority of biotitic lamprophyres studied in the minette class. 

 In consideration of the abundant presence of augite the rock may 

 be termed an augite-minette. 



Minette, Senator Stewart Mine, Kellogg. — A specimen of very 

 dense lamprophyric rock from the Senator Stewart mine, in Dead- 

 wood Gulch, near Kellogg, is intensely altered as seen in thin section 

 and only the original texture is preserved in the secondary minerals. 

 Apparently it was much like the glassy minette from the Marsh 

 Mine. It shows the usual fabric of biotite laths in what appears to 

 have originally been an alkalic glass, but which is now deep green 

 in color, and almost opaque. It is dotted with small grains of iron 

 ore. Large scattered fragments of broken and irregular outline are 

 now quartz, but appear to have originally been augites. 



Oli/vine-augite-niinette, Kellogg. — On the north side of the river, 

 on the Kellogg- Wall ace road, just east of Kellogg, there are exposed 

 a number of small lamprophyre dikes. These are directly across 

 the valley from the Bailey's Pond locality described below, and in 

 the field were supposed to represent the continuation of the same 

 dikes. Microscopic study of the largest and freshest dike at this 

 locality, however, shows the mineralogic composition to be entirely 

 unlike anything exposed on the south side of the river. There are at 



