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II. Rocks of Sedimentary Origin. 



(a) Limestones. 



The limestones in this Laurentian area are very 

 thick, and underlie a large part of it. 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 invaded 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 to repre- 

 sent on the map the gradual transition of the comparative- 

 ly unaltered blue limestones into the coarsely crystalline 

 white marble. This, however, takes place by the develop- 

 ment 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 

 and there through the 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. 



(b) Quartettes'. 



Quartzite is not common in this area, the most ex- 

 tensive development being that which occurs as a band 

 crossing the township of Monmouth. It is found inter- 

 stratified with crystalline limestones and rusty-weathering 

 gneisses of sedimentary origin. 



