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 abo-i-e, ]3ut much altered. Its triclinic feldspar is changed 

 in part to scapolite, in part to saussurite. Its hornblende, of 

 which tliere is comparatively little present, is largel}^ 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. The}' contain much twinned 

 hornblende and accessor}' biotite. Numl^er 15 differs only from 

 7 in containing more sphene and apatite. 



Number 22 is another diorite much like the last, l)ut whose 

 hornblende has a poikilitic structure, being mottled with plagio- 

 clase crystals. This sjiecimen also contains another mineral 

 now wholly decomposed and undeterminable. It somewhat re- 

 sembles biotite with calcite lenses, but it may once have been 

 corclierite, as it greatly resembles the altered form of that min- 

 eral described and figured l)y the writer in a granite from the 

 Black Forest, in Baden.f The groundmass of this rock ciMisists 

 of nearly idiomorphic plagioclase, much altered. 



Number 44 is a diorite Avhich 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 ap})ears 

 as a fine mixture of allotriomorphic green horn1>lende, plagio- 

 clase and sphene. This is evident!}' a dike rock. 



Quartx-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 Gushing near the foot of the 

 Muir glacier. These are somewhat luore acid rocks than the 

 quartz-free diorites, and are free from pyroxene. They contain 

 either biotite or green hornblende, or both, in varying amounts. 

 Thoir feldspar is a much striated and almost wholly idiomorphic 

 plagioclase, with a finely developed zonal structure. The quartz, 

 which is not particularly fjbundant, 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, pi. iii, fig. 1. 

 fNeues .Tahrbnch fur Min., Beil. Bd. ii, 1883, p. 598, taf. 12, fig. 1. 



10— Nat. Gkog. Mar., vol.. TV, 1892. 



