T. S. Hunt on Lithology. 181 
The granitoid dolerites of the Montreal group, containing 
coarsely crystalline augite and olivine, break through the Lower 
Silurian strata; and portions of these two minerals, probably 
derived from these intrusive rocks, are found in the dolomitie 
conglomerates near Montreal, which in some cases inelude masses 
of Upper Silurian limestone, and are cut by dikes of a fine- 
sang dolerite. These, which perhaps correspond to the newer 
ikes of the same rock at Grenville, show that there were at 
least three distinct eruptions of dolerite,—one during the Silu. 
tian period, one before it, and another after it. The trachytes 
of Montreal and Chambly appear to be still more recent, and to 
traverse these newest dolerites. 
The trachytes of Brome and Shefford seem to constitute a 
group apart; but the diorites of Yamaska and Mount Johnson, 
although similar in aspect, differ widely in chemical composi- 
tion. Facts are still wanting to establish the geological age of 
these intrusive masses, The different dolerites, which are related 
ferent diorites or the different trachytes of this vicinity are con- 
taneous. Nor, on the other hand, should even great dis- 
Cordances in chemical or mineralogical constitution be n ily 
Tegarded as establishing a difference in the age of eruptive rocks, 
Evidence to the contrary of this is seen in the contiguous and 
intermingled masses of black pyroxenite and gray feldspathic 
dolerite in Mount Royal and Montarville; and it is not improb- 
eee the olivinitic dolerite, which is associated wi ese, 
May 
firsy part of this paper, the various intrusive rocks are only dis- 
Placed sediments of deeply-buried and probably unconformable 
Strata, it will readily be conceived that plastic masses of very 
h 
an 1000 feet above the present level of the plain, appear 
_ “qually solid and crystalline with their bases, implies the re. 
_ Moval by denudation, since the eruption of these masses, of a 
_ . i¢kness of sedimentary strata much exceeding their present 
height. This denudation must however have taken place before 
a 
