18G4.] T. STERRY HUNT ON LITHOLOGY. 185 



Montirville ; and it is not improbable that the olivinitic dolerite 

 which is associated with these, may be contemporaneous. If. as 

 has been maintained in the first part of this paper, the various 

 intrusive rocks are only dis laced sediments of deeply-buried and 

 probably unconformable strata, it will readily be conceived that 

 plastic masses of very unlike characters may be ejected simulta- 

 neously along a line of disruption. 



The various intrusive masses of the Montreal group which have 

 been here describeil, appear, from their compact and crystalline 

 structure, to have been displaced and consclidated under the pres- 

 sure of a considerable mass of superincumbent strata. 'I he fact 

 that even their summits, which are in some cases more than 1000 

 feet above the present level of the plain, appear equally solid and 

 crystalline with their bases, implies the removal by denudation, 

 sin -e the eruption of these masses, of a thickness of s aiment ry 

 strata mu -h exceeding their present height This denudation 

 must however have taken place before the eruption of the later 

 trachytes and dolerites ; since the dolomitic conglomerates, which 

 enclose the fragments of the olivinitic dolerite and of Lower 

 and Upper Silurian rocks, repose unconformably upon the Lauren- 

 tian 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. If then, as is 

 probable, the expos re by denudation of the whole < f the eight 

 hills which have been described, took place at one e och, these are 

 all shown to have a greater antiquity than the trachytes and the 

 dolerites, which traverse the conglomerates. The tine-grained am' 

 earthy trachytes of Mo treal 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 irom among these intrusive 

 masses is a fact worthy of notice. Quartz has not yet been detect- 

 ed in th feldspathic rocks of Brome and Shefford ; although, as 

 above mentioned, the base of the feldspathic porphyries of Chambly 

 and Shelburne, contains a slight excess of silica. The granitic 

 rocks of Shipton, and of St. Joseph on the Chaudiere apfear to 

 be indig nous masses, belonging to the strata of the Quebec group; 

 but the higher fossiliferous iormations to the east of the Notre 

 Dame lountains, are traversed in various places by veins and 

 great masses of intrusive granite, as in Stanstead, Barford, and 



