434 INTRUSIVE ROCKS OF THE DISTRICT OF MONTREAL. 
composition of the voleanic rocks of Iceland and Teneriffe, and Scott 
has lately described a coarse-grained diorite from near Bogoslowsk 
in the Urals, which contains a feldspar of specific gravity 2.72, com-~ 
posed of silica 46.79, alumina 33.16, peroxyd ofiron 3.04, lime 15.97, 
potash 0.55 ; soda 1.28=100.79. It is associated with a greenish- 
black aluminous hornblende, containing some soda and titanic acid, 
together with a little mica and some quartz. (Phil. Mag. (4,) Xv. 
518). Quartz was also observed by Delesse in the orbicular diorite 
of Corsica, the feldspar of which contains according to him silica 
48.62, and lime 12.02, approaching to anorthite in composition. In 
all of these feldspars however, the proportion of silica is somewhat 
greater than in pure anorthite, which contains only 43.2 per cent. of 
silica. J have already in a previous Report discussed the question 
of the composition of these feldspars, and my reasons for regarding 
them as mixtures of two or more species. (Report for 1853-56, p. 
383, and Phil. Mag. (4) ix. 262.) I may here call attention to my 
analysis of the Bytownite of Thompson from near Ottawa; this is a 
granular feldspar, forming with occasional grains of hornblende a 
diorite, and having a specific gravity of 2.732, which in my Report 
for 1850, p. 39, I described as an impure anorthite. Its analysis is 
for comparison placed along side of that of the feldspar of the 
Yamaska diorite, and marked B. 
Mount Johnson or Monnoir, is composed of a diorite which im 
general aspect greatly resembles that of Yamaska except that it is 
rather more feldspathic ; the finer grained varieties are lighter colored. 
and exhibit a mixture of grains and small crystals of feldspar with 
hornblende, brown mica and magnetite. Frequently however the 
rock is much coarser grained, consisting of a mixture of feldspar 
grains with slender prisms of black hornblende often half an inch 
long and one-tenth of an inch broad, and numerous small crystals 
of amber colored sphene. 
In this aggregate there are imbedded cleavable masses of the feld- 
spar often an inch long by half an inchin breadth. At the southern 
foot of the mountain large blocks of the coarse grained diorite are 
found in a state of disintegration, affording detached crystals of 
feldspar with rounded angles, and weathered externally to an opaque 
white from partial decomposition. Near the base of the mountain.a 
coarse grained variety of the diorite encloses small but distinct 
