﻿Vol. 64.] LATJRENTIAK SYSTEM IN EASTERN CANADA. 137 



recrystallized. They have, therefore, an allotriomorphic structure,, 

 ■with a tendency of certain of the constituent minerals to elongate 

 themselves in the direction of the original bedding. While quartz, 

 felspar, and biotite are among the constituents present, the miea 

 is usually more abundant than in the granite-gneisses ; but, in 

 addition to these, garnet, sillimanite, graphite, and pyrite are very 

 frequently present, the last-named mineral giving rise to a pre- 

 vailingly rusty colour on the weathered surface. These gneisses 

 occur in the form of well-defined beds, and are usually found inti- 

 mately associated with the limestones. They resemble in many 

 respects the hornstones which are found . in granite contact-zones, 

 but are rather more coarsely crystalline than is usual in this class 

 of rocks. 



y. The Amphibolites. 



Intimately associated with these sedimentary gneisses and the 

 limestones on the one hand, and with the gabbros and diorites on 

 the other, is another class of rocks which are grouped under the 

 name ofamphibolite. While many varieties of these rocks occur 

 in the area, differing considerably one from the other in appearance, 

 they have as common characteristics a dark colour and a basic com- 

 position. Quartz, which is one of the commonest constituents in the 

 gneisses, is absent, or is present only in very small amount ; while 

 hornblende and felspar, the latter chiefly plagioclase, are the main 

 constituents of the rock. Pyroxene or biotite often replaces the 

 hornblende in part. A pale-green colour has been used in the 

 Bancroft sheet to represent the amj)hibolites, which colour serves 

 well to indicate the relation of certain varieties to the gabbro and 

 diorite-intrusions of the area, represented by a deeper shade of 

 green. 



These rocks underlie large areas, as will be seen from the accom- 

 panying map (PI. XIII). They also occur so intimately associated 

 with certain developments of the limestones, in the form of inter- 

 bedded layers, that these limestone-amphibolite occurrences have 

 been mapped separately. In places, the sedimentary gneisses also 

 fade away into occurrences of amphibolite when traced along the 

 strike. Masses of amphibolite too, as has been mentioned, abound 

 as inclusions throughout the granite of the bathyliths. 



These amphibolites furthermore are not peculiar to this area, but 

 occur abundantly everywhere in the Laurentian. They have always 

 proved to be one of the chief difficulties in the way of a correct 

 understanding of the geology of this system, seeing that it has been 

 impossible to do more than indulge in conjectures concerning their 

 origin. The same difficulty has been met with in the case of these 

 and allied rocks occurring elsewhere, as, for instance, the trap- 

 granulites of the Saxon Granulitgebirge or the amphibolites of 

 the crystalline complex of certain portions of the Alps, the origin 

 of which remained in doubt while that of the rocks wherewith they 

 are associated had been definitely determined. 



