ORIGIN OF AMPHIBOLITES OF LAURENTIAN AREA 3 
in the associated gneisses, is absent or is present only in very small 
amount. Pyroxene or biotite often replaces the hornblende in part. 
The amphibolites are sometimes rather coarse, but usually medium 
or fine in grain, and they possess as a general rule a more or less 
pronounced foliated structure. ‘They occur, as has been mentioned, 
associated, on the one hand, with the gabbro or diorite intrusions and as 
inclusions abounding throughout the granite of the bathyliths, and 
on the other hand they are often so intimately associated with certain 
developments of limestone in*the form of interbedded layers that it 
has been found necessary to map the two rocks together and to desig- 
nate them by a single color. 
Two of the more common varieties of these amphibolites which 
occur associated with the limestones were, during the course of 
mapping, designated as “feather amphibolite” and “granular 
amphibolite.”’ The first of these always occurs in thin bands inter- 
stratified with the limestone and derives its name from the curious 
feather-like development displayed by the large skeleton crystals of 
hornblende or pyroxene which are developed in the plane of the 
stratification of the rock and which give to the rock a striking appear- 
ance when it is split in this direction. The granular amphibolite, 
which also frequently occurs as heavy bands in the limestone, is 
of a finely granular character, without any very distinct foliation, 
and on the weathered surface presents a uniformly minutely speckled 
appearance, owing to the intimate admixture of minute grains of 
hornblende and feldspar. 
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As the result of a very careful examination, it is possible to prove 
conclusively that in this area the amphibolites have originated in 
three entirely different ways, the resulting rocks, although of such 
diverse origin, often being practically identical in appearance and 
composition. ‘This remarkable convergence of type, whereby rocks 
of widely different origin come to assume a practical 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. 
These three modes of origin are as follows: 
1. By metamorphism and recrystallization of impure calcareous 
sediments. 
