28 ANNALS NEW YORK ACADEMY OF SCIENCES 



The chief transition of the diorites is marked by the entrance of hyper- 

 sthene and enstatite, which leads to the hornblende norites with the de- 

 crease of biotite and to gabbro and biotite augite norite, with the entrance 

 of augite. The latter, however, is rare, and in only a few of the diorites 

 was either of these minerals noticed. In one case, an approach to a 

 diabasic texture was observed, but this reversal of the normal order is 

 uncommon. 



The alteration of the diorites is a very salient characteristic. Aside 

 from the ordinary weathering to chlorite, kaolin and epidote, sericite was 

 noticed in several specimens along the schist contact at the north end of 

 Lake Meahach. In these rocks, moreover, and in those taken from simi- 

 lar positions, a very perceptible amount of strain was noticed. The feld- 

 spars showed pronounced mortar structure and wavy extinction, and the 

 biotite was twisted and bent. The same phenomena are apparent in less 

 development in most of the diorite slides examined; either because of 

 their location, or on account of an inherent weakness in the rock itself, 

 the metamorphism of this type is more evident than it is in the case of 

 any other member of the series. 



Gabbro 



Gabbro is somewhat akin to the last mentioned type, but unlike the 

 diorite it is areally unimportant. Only one important area was found, 

 situated on the northern border just to the east of the syenite area there 

 and lying between the biotite augite norite and the mica schist. Its posi- 

 tion is thus analogous to that of the syenite, the accompanying norite here 

 carrying augite, whereas that which adjoined the syenite carried merely 

 biotite. Moreover, the syenite carries a small quantity of augite, which, 

 with other characters emphasized below, would seem to indicate its close 

 genetic relationship with this gabbro. The syenite and gabbro very pos- 

 sibly adjoin ; but since no outcrops are available for a space of half a mile 

 between them, it was deemed best to map them as found. 



The rock of this area is dark gray and micaceous, closely resembling 

 the syenite, although the short stout augites may be distinguished on close 

 examination. The grain is medium fine and the rock quite fresh, al- 

 though well jointed. In thin section, it appears that the plagioclase is a 

 basic andesine, but that nearly a third of the feldspar is unstriated and in 

 rounded irregular grains, indicating orthoclase. The biotite is abundant 

 and characteristic, with pleochroism from golden yellow to brownish 

 black. The augite is green, non-pleochroic, and shows extinction angles 

 from 45° to 50°. In basal section, it shows cleavages at 89° and gives a 

 good positive axial bar. Only one grain of hypersthene is visible. Apa- 



