154 REPORT ON THE PETROLOGY OF THE DOLERITES 



following the British usage adopted by Dr. Prior, they are here termed quartz-dolerites. 

 In addition there are quartzless and other types of dolerite. Throughout the proportion 

 of the felspar to the pyroxene is fairly constant, the former being slightly, more rarely 

 strongly, in excess. 



The plagioclase as a rule is subidiomorphic, occurring in thick platy forms, sometimes 

 with well-developed terminal faces. Its greatest extension is in the 010 plane. The 

 twinning is, as remarked by Dr. Prior, almost always after the Carlsbad law, less 

 commonly on the albite plan. Pericline twinning is infrequent, and is most noticeable 

 on the larger crystals. A single example was noticed of Manebach twinning, the crystal 

 being also twinned on the albite and Carlsbad laws. Zoning is common. When it is 

 well developed, the kernels of the plagioclase crystals, which occupy about half their 

 area, are very basic, approximating in composition to Ab 5 An 95 . Successive zones 

 rapidly decrease in basicity, the change being most rapid near the periphery. The 

 outermost zone has the composition Ab 65 An 35 , but from it still more acid extensions 

 may pass out into the micropegmatic material. There is never any reversal of the 

 change, an outer zone being invariably more acid than an inner one, showing an unin- 

 terrupted change of conditions in the magma during consolidation. It is rarely that 

 there is no sign of zoning ; the most nearly homogeneous crystals have the composition 

 of basic labradorite. As a rule the plagioclase is free from apatite and quite fresh. 

 The first alteration product to appear is chlorite, which is brought into the cracks of 

 the felspar by solutions deriving their material from the pyroxene ; the purely felspathic 

 decomposition product, sericite, is seen in a few instances only. Occasionally there is a 

 development of very fine cavities filled with liquid, but it is not certain that these are 

 of secondary origin. 



The pyroxenes of these rocks present many features of interest. Both the rhombic 

 and monoclinic pyroxenes are present in abundance, and these are developed in several 

 ways. In certain rocks they occur separately as bronzite or enstatite and augite. From 

 these there are passage rocks into those in which the two minerals occur together 

 in regular or irregular micrographic intergrowth. A third manner of joint oc- 

 currence is in isomorphous mixture with each other, forming the peculiar enstatite- 

 augite. 



In some rocks, in particular that from the moraine on the Knob Head Mountain, 

 in the Ferrar Glacier valley, the enstatite under crossed nicols has a peculiar appearance, 

 recalling that exhibited by anorthoclase. When the vertical axis is placed parallel to 

 the plane of the polariser it isseen that the extinction is slightly undulose, and there 

 appear a number of exceedingly minute needle-like portions that are not in exact extinc- 

 tion ; this is possibly due to an almost submicroscopic multiple twinning. The bronzite 

 shows the characteristic pinkish to greenish pleochroism, but in no instance is there 

 any schiller structure. When rhombic and monoclinic pyroxene occur separately the 

 rhombic seems rather the more nearly idiomorphic, though both may be ophitic. When 

 they are intergrown the enstatite or bronzite forms the matrix, and small strips of augite 

 may be scattered about in it quite irregularly, and without any definite orientation or 

 shape. In other cases there is a definite orientation ; the augite may form two sets 

 of strips in optical continuity with each other, and elongated parallel respectively to 

 the basal plane and vertical axis of an augite skeleton crystal ; and this augite skeleton 

 crystal, composed thus of rods, plates, and hook-like pieces, is oriented in the rhombic 

 crystal so that its vertical axis coincides with that of the latter. When the augite 

 skeleton is twinned, the two sets of stripes parallel to the basal planes form a herring- 

 bone pattern in the rhombic crystal. In some instances there may extend from a 

 twinned augite kernel thin lamellae of augite, running parallel to its basal plane out 

 into the surrounding rhombic pyroxene (Plate I, Fig. 2). This intergrowth of pyroxenes, 



