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mentary gneisses also fade away into occurrences of 

 amphibolite when traced along the strike. Masses of 

 amphibolite also abound as inclusions throughout the 

 granite of the batholiths. 



These amphibolites 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 encountered 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 were associated had 

 been definitely determined. 



Two of the more common varieties of these amphibolites 

 are represented by special designations on the map. One 

 of these, which has been termed "feather-amphibolite," 

 always occurs in thin bands interstratified with limestone, 

 and derives its name from the curious feather-like develop- 

 ment displayed by large skeleton crystals of hornblende 

 or pyroxene which appear on the plane of stratification 

 of the rock, to which they give a striking appearance 

 when it is split along this direction. The other variety 

 of amphibolite, which also frequently occurs as heavy 

 bands in the limestones, is of a finely granular character 

 without very distinct foliation. On weathered surfaces 

 it presents a uniformly, minutely speckled appearance, 

 owing to the intimate admixture of the minute grains of 

 hornblende and feldspar. On this account, during the 

 prosecution of the field-work, this variety was designated 

 as 'the pepper-and-salt amphibolite', and in the legend 

 of the Bancroft sheet it is designated as granular amphi- 

 bolite. 



Still other varieties differ from this granular amphi- 

 bolite, in being somewhat coarser in grain or less regular 

 in composition. 



As the result of a very careful examination, it has been 

 possible to prove conclusively that in this area the amphi- 

 bolites have originated in three entirely different ways, 

 the resulting rocks, although of such diverse origin, often 

 being identical in appearance and composition. This 



