INTRUSIVE ROCKS OF THE DISTRICT OF MONTREAL. 429 
earbonate of magnesia 3°58, carbonate of iron 3°82=19-00; a small 
portion of these bases was perhaps united with the alumina m a sili- 
cate. The insoluble residue gave as follows : 
I 
SILPCA NG esr 3 Seren re ete a lolets! scale live -c\u-aoa nia, at siahamiaene atotalee 61°62 
PAUTTITNITTVAy, Meta Meenave er eyane ate Siete siete iat cc aiabccolNeiaie elahtere 21-00 
UGANITC Hh Norn etevsaeret. afeteloleretaele lel city o mlel olelellefeie| share Reh e oitts 2°69 
Ma cnesiastege <tc etiaicfele ss athlete ata's Siac yeaa a (traces) 
PAG Ursa ip eer ater reeetaN eV Od Al fai (a Ja o¥s} 0\010)'s usnes Ha ate hen tone 4.66 
SEL ap hen ccyore yal eleneen ete das chats: Nieto) sje! 6 | Void: o lei eneupiaronatelstetatsns 5°85: 
Volatile, ...... emer tferere laces opete\ ciel sfosl'simictalacateuanenstateh pratt 2°37 
97°69 
It will be seen that this residue is near to orthoclase, or rather to 
oligoclase in composition ; as I have suggested in a previous Report, 
the decomposition of a portion of the feldspar, which has been con- 
verted into a hydrated silicate of alumina with loss of the alkalies 
and a portion of silica, will explain the presence of water and an 
excess of alumina, not less than the deficiency of silica and alkalies 
in the feldspathic matter of the more earthy of these trachytes. 
These trachytic rocks occur in dykes cutting the dolerites and 
melaphyres of the Mountain of Montreal, and constitute the little 
island known as Moffatt’s Island, but the most remarkable exhibition 
of them is met with in the mountains of Brome and Shefford. The 
former occupies an area of about twenty square miles in the township 
of Brome and the western part of the township of Shefford, and con- 
sists of a great mass of trachyte risimg mto several rounded hills, 
of which Brome and Gale Mountains are the principal, and may 
have an elevation of about 1000 feet above the surrounding plain, 
from which the intrusive rock rises boldly. It shows divisional 
planes, giving it the aspect of stratification, and is divided by other 
joints into rectangular blocks. Another similar mass, covering an 
area of about nine miles, is met with in the township of Shefford a 
little to the N.W., and distant in the nearest point only about two 
miles from the last. These masses of rock, as Sir W. E. Logan has 
shown in his Report for 1847, break through the slates and sandstones 
of the upper portion of the Hudson River group, which in that 
vicinity, although on the confines of the metamorphic region, are but 
little altered. 
The rock of these two mountainous areas presents but very slight 
differences, being everywhere made up in great part of a cleavable 
feldspar with small portions of brownish-black mica or of black horn- 
