a 
INTRUSIVE ROCKS OF THE DISTRICYr OF MONTREAL. 431 
A specimen from the south side of Shefford Mountain was next 
examined. A little above the place where it was collected, the rock 
was a coarse greyish-white feldspar with a little black mica, and 
closely resembled that just described, but the portion selected contain- 
ed a little black brilliant hornblende in crystalline grains about the 
size of those of rice, with very small portions of magnetite and yellow 
sphene, disseminated ina base, which although completely crystalline, 
was more coherent and finer grained than that of Brome, rarely 
exhibiting cleavage planes more than one-fourth of an inch in length. 
Its colour was yellowish-white, and it was sub-translucent with a some- 
what pearly lustre. Fragments of the rock gave a specific gravity of 
2°607—2°626—2°657. By crushing and washing the mass, the white 
feldspar grains were separated from the heavier minerals, and had 
im powder a specific gravity of 2-561. 
The composition of this feldspar is almost identical with that from 
the trachytes of Brome and Chambly. For the sake of comparison, 
the analysis of the crystals from the latter is subjoined. (A.) See 
Report for 1856, p. 486. 
Analysis gave for the feldspar of Shefford : 
IV. A. 
STC ees) srt arrears ee wheal ohaler ame varetaletians 65°15 66°15 
JANINE Ain OnE OOOO ORC O OOO UNOOn 20°55 19°75 
AIT SORA OCI CD SOOO OT Ocoee aac ne ots 95 
PEGs cree hat o: ayevsaicv evs conalia rat svatershaterouateteteletst ete 6°39 7°53 
Sodat cs .c adits B apsanchevaierslaleleratoia Aeralciereletacat eve 6°67 5°19 
WGLEGILCS emia sito sc cie we ot cgec eee cco eats *50 ‘5b 
99°99 100-1 
Going westward from the mountains of Brome and Shefford, which 
from their proximity and their identity of composition'may be looked 
upon as forming but one great trachytic mass, we meet with a series’ 
of intrusive masses, less extensive, but similar in attitude, and which, 
as Sir Wm. Logan has remarked, are placed along the line of an anti- 
clmal, traceable as a gentle undulation for 180 miles across the country 
as far west as the Lac des Chats on the Ottawa. The hills lying to the 
west of Brome and Shefford are in the order of their succession, 
Yamaska, Rougemont, Belceil, Montarville, Mount Royal and Rigaud, 
all of which are intruded through Lower Silurian strata. A few 
miles to the south of Belceil is Mount Johnson or Monnoir, another 
intrusive mass, which although somewhat out of the range of those 
