206 PETROLOGY OF ROCK COLLECTIONS 



outer shell 1-2 cm. diameter in which the quartz particles are cemented by growth - 

 enlargment of the grains themselves, and of an interior filling 4-6 cm. diameter of 

 quartz grains cemented by calcite in optical continuity. Obviously the localisation 

 of alkaline substances or calcite has determined the formation of these concretions. 



One fragment of the Beacon Sandstone formation from the Moraine, East Fork of 

 the Ferrar Glacier, is loaded with calcite in optical continuity. A similar Fontainbleau 

 Sandstone occurs directly above the Permo -Carboniferous tillite at Hallett's Cove, South 

 Australia.* It appears to be commonly the case that mechanical sediments of glacial 

 periods become loaded with calcite or other chemical precipitate. 



Certain specimens from the East Fork of the Ferrar Glacier, obviously belonging 

 to the Beacon Sandstone horizon, are highly felspathic. They contain quartz, ortho- 

 clase, acid plagioclase, muscovite, biotite, pink garnet, rutile, and iron ores. This is 

 a transition towards the typical psammite of the Beardmore Glacier area, which is an 

 arkose quite remarkable in character and deserving a special description. There seems 

 to be no reason to doubt its identity as an example of the Beacon Sandstone formation. 

 Its peculiar characters are just such as would be expected from rapid disintegration 

 under semi-glacial conditions. Carbonaceous material appears along the bedding 

 planes of some of the specimens, just as is the case in the specimens from the Ferrar 

 Glacier. 



The following description refers to the more massive varieties free from organic 

 matter. 



Arkose from the Upper Glacier Depot, Beardmore Glacier 



The hand-specimen is an even-grained sand-rock of an ash-grey colour. A large 

 proportion of felspathic material, partly kaolinised, is present. Particles of mica are 

 not infrequent, most abundantly distributed along the bedding planes ; this endows 

 the rock with a ready fissility. With the aid of a pocket-lens particles of pink garnet 

 are rendered obvious. 



A microscopic examination showed it to be composed of particles averaging in the 

 prepared slide 0-12 mm. diameter."}" (Figs. 1 and 2, Plate III). The grains have 

 angular and sub-angular boundaries with splintered edges. 



Quartz is the chief constituent. Under crossed nicols most of the particles exhibit 

 strain shadows or are seen to have suffered crushing by granulation. Frequently a 

 rejuvenation is evidenced by clearer border zones. Abundant inclusions, chiefly 

 rutile needles, are present. The felspars are predominantly orthoclase, microcline, and 

 anorthoclase, though a minor proportion show albite lamellae and agree in optical 

 properties with albite and oligoclase. They have a dusty appearance in transmitted 

 light due to partial kaolinisation, and certain grains show chloritic changes. Under 

 crossed nicols the effects of strain are again noted. Mica, both muscovite and biotite, 

 is present sparingly, in small particles, usually partly converted to chlorite and often 

 much bent. Apatite in occasional inconspicuous grains. Particles, originally ilmenite, 

 now altered to leucoxene, are frequent. Pink garnet in small quantity, often embedded 

 in the felspar individuals, is scattered throughout the section. Rutile is present as 

 microscopic hair-like inclusions in the quartz and rarely as isolated grains. Sphene 

 grains are amongst the rarest of the accessory constituents. 



A Rosiwal determination of the approximate mineral composition by volume 

 gave : 



* " Mineralogical Notes," Trans. Roy. Soc.S. Aust., vol. xxxi, 1907, p. 119. 

 t Note that in a granular rock the average absolute diameter of the grains is about twice that 

 obtained by averaging the diameters observed in the microscope slide. 



