90 DISCOVERY REPORTS 



Tremolite-epidote-gree7istone. The principal minerals, as disclosed by thin sections, are tremolite, 

 clinozoisite-epidote, chlorite, quartz, and albite. They are arranged in bands or elongated folia parallel 

 to an ill-defined slaty cleavage which, in two or three of the sections, develops into a phyllitic or 

 schistose structure. The cleavage planes are frilled and puckered by an imperfect strain-slip. The 

 bands consist of one or two of the above minerals to the almost complete exclusion of the others. 

 Folia consisting mainly of tremolite and clinozoisite or epidote are preponderant. 



The tremolite occurs as colourless to pale green fibres, needles, prisms and plates, often arranged 

 in parallel position or with a slightly divergent, sheaf-like structure. It has a good cross-fracture and 

 longitudinal cleavage, although the typical prismatic amphibole cleavage is rarely seen. The extinction 

 is at io~20° to the cleavage direction [c). Its elongation is positive in sign, distinguishing it from the 

 colourless variety of pargasite (edenite). Both epidote and clinozoisite are present. Epidote is the 

 most frequent associate of the tremolite. It is of yellowish brown colour, and has usually undergone 

 considerable alteration converting it into a greyish cloudy material (leucoxene.?). This material forms 

 ragged areas or, in the more highly cleaved types, it is drawn out into streaks and lines. It is possible 

 that some of this material may represent altered sphene. Colourless clinozoisite occurs mainly as well- 

 shaped crystals associated with quartz and albite in lenticles which may be partly of secondary origin. 



Chlorite of the pale green variety with ultra-blue polarization occurs in irregular pods or stout 

 lenticles. It is not abundant and, in a few places, appears to be growing at the expense of tremolite 

 and epidote. Quartz, always with undulose extinction, is abundant in some lenticles and bands, and 

 is associated with a little untwinned or simply twinned albite. Finally, in a few of the less altered 

 rocks, very slender microlites of plagioclase (oligoclase.'') can be detected. Discussion of the original 

 character of this somewhat unusual greenstone is deferred to the section dealing with its chemical 

 composition (p. 91). 



Qiiartzite. This is a hard, yellowish, well-cemented sandstone or semi-quartzite. In thin section 

 it is found to consist mainly of quartz and feldspars (plus alteration products) in roughly equal 

 proportions. All the grains are angular and fit together like the stones in macadam. Only a few of the 

 quartz grains show undulose extinction. The feldspar is easily distinguished by its turbid appearance. 

 It includes soda-orthoclase and albite in about equal proportions. Many of the grains are com- 

 paratively fresh despite their turbidity, but others are completely altered to sericite and crystalline 

 kaolinite. These alteration products have insinuated themselves into fissures in the quartz grains 

 and between the grains, thus acting as a cement which has filled all open spaces. In addition to 

 quartz and feldspar there are a few small grains of epidote, sphene, and iron ores, and rather more 

 abundant fragments of what appears to be the ground-mass of dense acid igneous rocks like felsite 

 or rhyolite. In fact, the mineral composition of the rock suggests that it may have been derived from 

 the waste of rocks like the quartz-feldspar-porphyries which constitute the major part of a great 

 Porphyry Formation in Patagonia and Tierra del Fuego, and are also found in parts of West Antarctica 

 (see this Memoir, p. 75). The rock may thus be described as quartzitic arkose. 



Quartz-vein rocks. These are all mainly composed of white quartz with films of a chloritic mineral. 

 In thin section one of them shows quartz, albite, chlorite, and a little calcite, all intensely sheared and 

 crushed. The quartz has marked undulose extinction and in the albite the twinning lamellae are bent 

 and twisted. The chlorite is greyish green, and shows the common ultra-blue polarization; it is 

 occasionally quite isotropic. 



A second rock consists of intensely sheared and sliced quartz with some large crystals of greenish 

 brown epidote. While clearly later than the quartz, the epidote crystals have also been bent and sliced 

 by a movement in a diff'erent direction to that which first affected the quartz. The resulting fissures 

 have been healed by the infiltration of silica. No albite or chlorite occurs in this rock. 



