﻿136 PROF. p. DAWSON ADAMS ON THE [May 1 908^ 



IV. The Kocks of Sedimentary Origin. 

 The Limestones. 



The Hmestones in this Laurentian district are very thick, and 

 underlie a large part of the area. In their more altered form they 

 closely resemble those described by Logan in the areas examined 

 by him ; but to the south-east of the Bancroft sheet, where the 

 invading granite is less abundant and the alteration of the in- 

 vaded strata is correspondingly less pronounced, the limestones 

 appear in less altered forms, and eventually pass into fine-grained 

 greyish-blue varieties in which the bedding is perfectly preserved 

 and concerning whose truly-sedimentary character there can be 

 absolutely no doubt. It is impossible on the map to represent the 

 gradual transition of the comparatively-unaltered blue limestone 

 into the coarsely-crystalline white marble. This, however, takes 

 place by the development in the former of little strings or irregular 

 patches of coarsely-crystalline white calcite, usually following the 

 bedding-planes. These become larger and more numerous on going 

 north in the area towards the granite-intrusions, until eventually 

 the whole is transformed into a great development of white marble.. 

 Here and there through this marble, where the bodies of the rock, 

 are very thick, small remnants of the original blue limestone can 

 occasionally be found, as is indicated on the map, in the township 

 of Monmouth, 



Enormous bodies of nearly-pure limestone occur in many parts 

 of the area; but elsewhere this limestone is impure, owing to the 

 presence of grains of various silicates distributed through it, or to 

 the presence of little bands of silicates representing impurities in 

 the original limestones, which, under the influence of metamorphism, 

 develop into gneisses and amphibolites of various kinds. Where 

 these little gneiss-bands or amphibolite-bands become increasingly 

 abundant, the limestone passes over into paragneiss or into some 

 one of the varieties of amphibolite. 



The Quartzites. 



Quartzite is not common in this area, the most extensive develop- 

 ment being that which occurs as a band crossing the township of 

 Monmouth. It is found interstratified with crystalline limestones 

 and rusty-weathering gneisses of sedimentary origin. 



There is every reason to believe that these quartzites represent,, 

 in most cases at least, altered sandy sediments. 



The Gneisses of Sedimentary Origin (Paragneisses). 



These rocks differ distinctly in appearance from the foliated 

 granite-gneisses already described as constituting the bathj-lithic 

 intrusions. They are fine in grain, and show no protoclastic or 

 cataclastic structure, the original material having been completely 



