522 J. A. DRESSER IGNEOUS ROCKS OF EASTERN QUEBEC 



tact zone. Moreover, the sedimentary rocks through which the granites 

 and the diabase series have been intruded are generally very similar to 

 those surrounding the Monteregian hills ; in fact, the Hudson Eiver shales 

 are the only rocks of the latter region not found in the former. Hence it 

 would seem that, whatever the cause may be of the primary magmatic 

 differentiation, the magma which gave rise to the Monteregian hills was 

 primarily different from that which produced the other ■ rocks discussed, 

 with the partial exception already mentioned, of the later dikes. 



Summary 



In summary it may be said that the rocks of southeastern Quebec 

 present two petrographical provinces, and their differences are due to 

 primary differentiation — that is, to differences in the original magmas. 



I. (a) Porphyry-andesite series, extrusive and probably of pre-Cam- 

 brian age. 



(&) The diabase-serpentine group, inclusive in a large measure, rang- 

 ing in age from early Cambrian to late Silurian. 



(c) The granites, intrusive, late Devonian age. 



(d) Between these and the next province, in a measure bridging over 

 the gap between them or at least indicating that the extreme limit of 

 each has been reached, are the later dikes. 



II. The second province comprises properly only that unique group, 

 the Monteregian hills. 



Addendum 



For the completion of the investigation which has been briefly outlined 

 in this paper a large amount of field and laboratory work is yet necessary, 

 much of which has an economic importance that is not less than its value 

 to pure science. The later dikes are very little known, and yet the 

 occurrence of important deposits of copper in the northern part of the 

 district occupied by them has been known for fifty years. The granites 

 are a geologic unit concerning which we have very little knowledge and 

 of which correspondingly little use is made, while the pyrrhotites of the 

 diabase series promise economic results perhaps no less important than 

 the asbestos and chromic iron deposits, which also await detailed petro- 

 graphic investigation. 



