182 T. S. Hunt on Litthology. 
the eruption of the later trachytes and dolerites; since the dolo- 
mitic conglomerates, which enclose the fragments both of the 
olivinitic dolerite and of Lower and Upper Silurian rocks, re- 
ose unconformably upon the Laurentian and the various Lower 
Silurian strata, in such a manner as to show that these offered 
nearly their present distribution at the epoch of the deposition 
of the conglomerates. then, as is probable, the exposure by 
denudation of the whole of the eight hills which have been de- 
scribed, took place at one epoch, these are all shown to havea 
greater antiquity than the trachytes and the dolerites which 
traverse the conglomerates. The fine-grained and earthy tra- 
ehytes of Montreal are consequently far more recent than the 
crystalline ones of Brome and Shefford; with which, however, 
some of them agree in chemical composition. 
The general absence of granite from among these intrusive 
masses is a fact worthy of notice. Quartz has not yet been de- 
tected in the feldspathic rocks of Brome and Shefford ; although, 
as above mentioned, the base of the feldspathic porphyries of 
Chambly, and of Shelburne, contains a slight excess of silica. 
The granitic rocks of Shipton, and of St. Joseph on the Chau- 
diére, appear to be indigenous masses, belonging to the strata of 
the Quebee group; but the higher fossiliferous formations 10 
facts are in accordance with the theory of eruptive rocks devel 
oped at the commencement of this paper; and it would be easy 
_to extend the comparison to the intrusive diorites and dolerites 
about Montreal, and to show their resemblance with the stratified 
feldspathic the Labrador series. (Zhis Journal, [2], 
