﻿138 PROF. F. DAWSON ADAMS ON THE [May I908, 



Two of the more common varieties of these amphibolites are 

 represented by special designations on the map (PI. XIII). 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 development 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 (see PI. XII). 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 ; and, on the weathered surface, it presents a 

 uniformly, minutely-speckled appearance, owing to the intimate 

 admixture of the minute grains of hornblende and felspar. 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 amphibolite. 



Still other varieties differ from this granular amphibolite, 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 amphibolites have origi- 

 nated in three entirely different ways, the resulting rocks, although 

 of such diverse origin, often being identical in appearance and com- 

 position. This remarkable convergence of type, whereby rocks of 

 widely-different origin come to assume identity of character, explains 

 the difficulty which has been experienced up to the present time 

 in arriving at a satisfactory conclusion concerning their genetic 

 relations. 



(a) Some of these amphibolites result from the metamorphism 

 and recrystallization of sediments. To this class belong the 

 feather-amphibolites above described, which usually occur 

 in thin bands alternating with crystalline limestone, and are 

 evidently of like origin. They represent siliceous or dolomitic 

 laminae in the calcareous deposit. In many cases the bands of 

 crystalline limestone become thinner and less abundant, and the 

 composite rock passes gradually over into a body of pure feather- 

 amphibolite. Whether the granular amphibolite, which is also found 

 very frequently and over wide areas alternating with bands of 

 limestone, is in some cases of similar origin, it has not been possible 

 up to the present time to decide. 



(b) Certain granular amphibolites represent altered igneous 

 intrusions, for they are found in the form of dj-kes cutting across 

 the stratified white crystalline limestone, on the shores of Jack's 

 Lake in the township of Methuen. The limestones here dip at a 

 low angle to the south, and are excellently exposed in the form 

 of low cliffs about the side of the lake. The typical granular 

 amphibolite can be seen rising above the surface of the water in the 

 form of vertical dykes, cutting directly across the stratification of 

 the limestone. These, which are 1 to 2 feet wide, can frequently be 

 seen on reaching a certain bedding-plane to have been bent over in 



