156 REPORT ON THE PETROLOGY OF THE DOLERITES 



or in subradiating groups. It is difficult to determine whether these are quartz or not. 

 The intergrowth is identical in character with that forming the mesostasis of the enstatite- 

 augite bearing rock of Launceston in Tasmania, and in this Professor Ossan * considers 

 the clear strips to be andesine on account of their refractive index and birefringence 

 being identical with that of the outer zone of the plagioclase crystals, to which they 

 are sometimes parallel both in extension and optical orientation, Nevertheless a little 

 free quartz in irregular grains is sometimes associated with this intergrowth. 



Apatite is present in very small amount only, and occurs almost exclusively with the 

 micropegmatite. 



The general type of the phaneric erratics of Cape Royds and that at the pseudo-Cape 

 Irizar is semi-ophitic in texture, and is composed of basic zoned plagioclase, enstatite- 

 augite, and a varying amount of interstitial micropegmatite. In addition there may 

 be a small amount of biotite and ilmenite. In some instances the amount of micro- 

 pegmatite may be very great (see Plate I, Fig. 1). In another variety with a slightly 

 porphyritic habit both monoclinic and rhombic pyroxenes are present ; the latter is 

 the more abundant. Its pale pink to green pleochroism prove it to be bronzite. The 

 monocliuic pyroxene is the more ophitic in habit, and its optic axial angle is large. 

 A certain amount of pyroxene perthite is present regularly and irregularly developed. 

 A little biotite, ilmenite, and a fair amount of micropegmatite are also developed. In 

 a somewhat similar rock enstatite-augite occurs with the rhombic pyroxene, sometimes 

 in parallel growth. No. 1545 is strongly porphyritic. The bronzite crystals are up to 

 four millimetres in length ; the monoclinic pyroxene, enstatite-augite, is less abundant, 

 ophitic, and with a small optic axial angle ranging from 20° upwards. The plagioclase, 

 labradorite, is slightly zoned, and is present in small tabulae and a few phenocrysts. The 

 chemical composition of this rock is given on page 157 (No. I). 



The rock collected by Priestley from the Knob Head Mountain moraine in the Ferrar 

 Glacier valley is a most beautiful rock, and is quite distinct from that obtained from 

 the mountain itself by Ferrar. It shows in hand-specimen a cloudy white felspar matrix 

 studded with phenocrysts of yellow-brown enstatite. Under the microscope the plagio- 

 clase appears to be in very small crystals, though there are some larger grains. It is 

 basic labradorite and is slightly zoned. Here and there are scattered through it patches 

 of isolated grains of colourless augite, in optical continuity. The phenocrysts of enstatite 

 are sometimes four or five millimetres in length, and frequently exhibit the strained 

 anorthoclase-like appearance before described. They are not idiomorphic, but usually 

 are ophitically intergrown with felspar on the periphery, and may contain felspar laths. 

 Sometimes also they are grown with monoclinic pyroxene in parallel position, the latter 

 fraying out ophitically into the felspar (see Plate I, Fig. 3). 



No. P. 204 is an erratic from Cape Royds in which the ophitic structure is developed 

 to its highest degree. The augite has a large optic axial angle, is pale in colour, and 

 is commencing to alter into chlorite, which in a very few places contains a fibrous, radiat- 

 ing, colourless mineral with some of the optical properties of stilbite. A little ilmenite 

 is present, but no micropegmatite. 



No. P. 242 is the alteration product of a rock similar to the above, but differing in 

 the presence of a little biotite and quartz. The felspar laths are changing into sericite, 

 the augite is passing into chlorite carbonates and secondary mica. A little magnetite 

 and a few apatite needles are also present. 



With the phaneric dolerites may also be classed No. P. 252, a Cape Royds erratic. 

 It resembles the quartz dolerites in the abundance of the rhombic molecule, and the 

 presence of micropegmatite ; it differs in the comparative abundance of potash, as 



* " Ueber einen Enstatitaugit-fuhrenden Diabase von Tasmanien," Centralblatt fur Mineralogie, 

 1907, pp. 705-711. 



