National Geographic : 1892 Mar 21
Varieties of Diorite. 67 and the interior of its hornblende individuals is often granulated with quartz or albite, as in the Baltimore gabbro-diorites.* Saussurite-diorite.-Number 6 is a rock similar to those de scribed above, but much altered. Its triclinic feldspar is changed in part to scapolite, in part to saussurite. Its hornblende, of which there is comparatively little present, is largely changed to epidote and its biotite to chlorite. Considerable irregular areas of secondary quartz are also present. Numbers 7 and 15 are medium-grained diorites with nearly idiomorphic feldspar crystals. They contain much twinned hornblende and accessory biotite. Number 15 differs only from 7 in containing more sphene and apatite. Number 22 is another diorite much like the last, but whose hornblende has a poikilitic structure, being mottled with plagio clase crystals. This specimen also contains another mineral now wholly decomposed and undeterminable. It somewhat re sembles biotite with calcite lenses, but it may once have been cordierite, as it greatly resembles the altered form of that min eral described and figured by the writer in a granite from the Black Forest, in Baden.t The groundmass of this rock consists of nearly idiomorphic plagioclase, much altered. Number 44 is a diorite which differs from the others in being of a very much finer grain. In the hand-specimen it is dark green and quite aphanitic, while under the microscope it appears as a fine mixture of allotriomorphic green hornblende, plagio clase and sphene. This is evidently a dike rock. Quartz-diorite.-Closely allied to the foregoing diorites and differing from them chiefly in their content of quartz, are six specimens collected in situ by Mr Cushing near the foot of the Muir glacier. These are somewhat more acid rocks than the quartz-free diorites, and are free from pyroxene. They contain either biotite or green hornblende, or both, in varying amounts. Their feldspar is a much striated and almost wholly idiomorphic plagioclase, with a finely developed zonal structure. The quartz, which is not particularly abundant, occupies the interstices be tween the well-formed feldspar crystals as the augite does in diabase. It is very plainly the last product of crystallization. * Bulletin U. S. Geological Survey, no. 28, 1886, pl. iii, fig. 1. t Neues Jahrbuch fir Min., Beil. Bd. ii, 1883, p. 598, taf. 12, fig. 1. 10-NAT. GEoG. MAG., VOL. IV, 1892.
1892 May 15
1892 Feb 19