124 ROYAL SOCIETY OF CANADA 



minerals. Muscovite micas are rare, and in most cases we liud tne 

 third constituent mineral, mica or hornblende replaced by a chloritic 

 or other cryptocrystalline mineral of large magnesian content. So 

 far has this change to a hydrated condition gone, that the diorite dykes 

 have been serpentinized, and the quartzites, where the silicates were 

 abundant, have become a quartzose gneiss. 



There seems little doubt that this system, after it was deposited 

 and had been injected with igneous rocks, was buried and subjected 

 to the action of heated, silicated waters, and then re-elevated, before 

 the deposition of the next system. This implies the lapse of a con- 

 siderable interval of time between the Laurentian and the overlying 

 Huronian. More than this, the enormous amount of quartzites in the 

 gold-bearing rocks of Nova Scotia, of the next terrane, implies a large 

 continental area of rocks similar to those of this Laurentian system; 

 such would be required to provide the necessary material for the pro- 

 duction of the vast quantities of quartz sand required to build 

 up Ihe great mass of quartzites that characterizes the Maguma or Gold- 

 bearing series of Nova Scotia. 



We can scarcely look elsewhere for this great area of crystalline 

 rocks than to that part of the continental shelf of North America, now 

 submerged beneath the sea off the southwest coast of Nova Sootia. 

 That this may have been the source of the Maguma sands seems also 

 implied by Prof. L. W. Bailey's observations as to the existence of con- 

 glomerates in this formation in the southwest Atlantic belt of these 

 rocks in that province. 



As a resistant area of consolidated sediments the Laurentian area 

 at St. John is a good example. All its old intrusives stop short at 

 its borders, and the terrane shows an amount of metamorphism much 

 greater than the adjoining terranes that succeeded it. We therefore 

 may regard it as a terrane which had been formed, consolidated and 

 metamorphosed at a date anterior to the Huronian terrane which rests 

 upon it. It formed a firm bamer to the earth-movements that were in 

 progress at a later day as the subsequent terranes are ridged up against 

 it, and its borders were the lines of important faults that continued- to 

 effect the geological structure for many ages subsequently. 



The syenites, gneisses and diorites, which occur in the Laurentian 

 rocks have been studied by Dr. W. D. Matthew,^ who found the syenites 

 to be "a coarse-grained quartzose granitic rock, with large porphyritic 

 crystals of pink weathering feldspar," and he classed it as an intrusive 

 quartz diorite, or hornblende granite. The gneisses are variable in 

 character, at times quite massive, but generally schistose, granular, 



* Intrusive rocks near St. John, N.B., by W. D. Matthew. Contrib. Geol. 

 Dep., Columbia Coll., No. XXII, p. 187. 



