204 PETROLOGY OF ROCK COLLECTIONS 



PSEPHITES 



The conglomerates are almost all referable to the Beacon Sandstone formation, 

 and usually represent pebbly bands interlaminated with the fine-grained sandstone. 

 It is safe to assume that the base of the series is a definite conglomerate horizon, as 

 coarse examples, likely to have come from such a situation, are met with in the recent 

 moraines. A specimen of such a conglomerate from the Knob Head Mountain Scree,* 

 west (Ferrar Glacier), is at hand. In it there is a general absence of anything but quartz, 

 of which the pebbles range up to 3 cm. in diameter, and are very firmly cemented 

 together. 



A conglomerate of a much later age appears amongst the material from the Stranded 

 Moraines, McMurdo Sound, and from Dry Valley, New Harbour. It is suggestive of a 

 recent age, and, as it contains fragments of the mainland dolerite, it is almost certainly 

 Tertiary or Post-Tertiary. Its constituents have accumulated under water and the 

 pebbles are not obviously glaciated. The constituents comprise biotite, muscovite, 

 felspar, crystal quartz, jasper, chert, and composite rocks ; amongst the latter are dolerite 

 and a pre-existing grit consisting chiefly of white and crystal quartz. Cementing these 

 there is an abundance of a light yellow mud consisting mainly of carbonates of lime and 

 magnesia. This rock appears to tally with that mentioned by Dr. Prior | as occurring on 

 the western promontory of Black Island. Its characteristics suggest an aqueo-glacial 

 origin and its distribution infers deposition in recent times in connection with the 

 affluent streams and ponded waters at the termini of the large mainland glaciers. 



PSAMMITES 



These are chiefly sandstones and arkoses, which are represented in varying degree 

 of metamorphism. By a study of the moraine -derived specimens it is not possible 

 to say how far the quartzites represent the Beacon Sandstone, or to what extent they 

 proceed from older formations. Amongst the metamorphic rocks are sandstones and 

 greywackes intruded by granite ; this infers a certain antiquity for the intruded 

 sediments. 



In the collection from the East Fork of the Ferrar Glacier is a very white fine- 

 grained quartzite ; between this quartzite and an almost pure marble, graduated steps 

 are represented by other specimens. These marbles are thought to be of Palaeozoic 

 age, and the inference is that quartzites, also of this age, exist. 



The facies of the Beacon Sandstone formation is so distinctive that one has no 

 hesitation in referring almost all the psammites to this formation. A collective des- 

 cription will be found most convenient. 



The Beacon Sandstone 



A wide geographical range is indicated by its appearance in all the mainland col- 

 lections. Amongst the collection made by Priestley in the Ferrar Glacier region it is 

 most typical and abundant. The Southern Party met abundance of the more fel- 

 spathic varieties between 83° 50' S. lat. and 85° 10' S. lat. A few fragments only were 

 met with by the Magnetic Pole Party, though the topographical features of the coast 

 ranges in that direction infer its continuance northward on a grand scale. 



The constitution of this rock varies through wide limits, from a nearly pure quartzose 



* For maps refer to vol. i of these Reports. 



t British Museum Reports on National Antarctic Expedition, 1901-4, vol. i, Geology, p. 139. 



