GREGORY.] 
QUARTZ-TRACHYTE. 
167 
be almost entirely ferrous carbonate, as is common in rocks of tin- 
type. 1 It will be noticed that if a series of rocks of this type is to be 
constructed from comparison of the analyses, the Maine rock is the 
most acid type so far described, although some silica should be deducted 
for the secondary quartz present. The chemical composition of the 
quartz-trachyte differs little from that of the rhyolites (see p. 155), 
but the appearance of the two hand specimens is distinctly different. 
The aplite-granites (VI) have also much the same composition, but? 
their structures, both macroscopically and microscopically, show char- 
acteristic differences. 
The percentages of mineral components present in the Quoggy Joe 
quartz-trachyte have been calculated from the analysis and found to 
be as follows: 
Mineral components of Quoggy Jar quartz-trachyte. 
Mineral. 
Per cent. 
Mineral. 
Percent. 
Quartz 
34.97 
27.86 
30.46 
3.88 
Kaolin 
1.66 . 
.50 
.44 
Orthoclase 
Chlorite 
Iron ore, etc 
Albite 
Siderite 
Total 
99.77 
Classification. — The term which should be applied to a rock of this 
character is not clearly evident. It belongs, however, to that variety 
of trachyte found in many places which is characterized by a white or 
creamy color, rough trachytic surface and appearance, a peculiar silky 
sheen, and a distinct mode of weathering. By this mode of weather- 
ing, which is rare for ordinary flow trachytes, but very characteristic 
for quartz- trachytes, the rock does not show gradations from weath- 
ered to fresh surface, but shows a thin dark outer zone which is 
suddenly replaced by the fresh white material within. Further! no re, 
so far as alterations can be considered characteristic, it is marked 
by the presence of siderite in place of calcite. Its microst incline is 
also characteristic in that it retains a typical trachytic structure of 
feldspar laths whose interspaces are tilled with quartz, and not the 
granular structure common in alkaline rocks so full of silica. Hunter 
and Rosenbusch named rocks of this habit bostonite, 2 thus distinguish- 
ing a new class of dike rocks, and taking the Marblehead Neck outcrop 
as the type, although at that place it is apparently a surface flow. 
Chemically and mineralogically bostonites are practically trachytes, 
except that soda orthoclase is unusually abundant and the dark sili- 
cates are very few or are altogether lacking. 
iBrogger, Das Ganggefolge des Laurdalits, p. 201. 
2Tscher. Min. u. Petrog. Mitth. 1890, p. 417. 
