252 AUSTRALASIAN ANTARCTIC EXPEDITION. 



A search of some of the literature of the quartz-dolerites reveals the fact that 

 the peculiar habit and curvature of the pyroxene has often been noted. It is true 

 there is no mention of it, for example, in Marker's description of the Carrock Fell 

 intrusion 1 , or Elsden's account of the St. David Head Rock series 2 , or in J. V. Lewis's 

 report on the diabases of New Jersey, U.S.A. 3 . 



But Teall, examining the dolerite of the Whin Sill 4 , noted a coarse-grained 

 variety occurring " only where the rock attains a very considerable development," in 

 which " crystals of pyroxene measuring an inch in length are not uncommon." The 

 crystals are flattened parallel to (100), and the cleavage planes are " bent and 

 undulating," pinacoidal twinning is present, also a fine basal striation, absent when the 

 rock is fresh but present in the most altered specimens. The coarser rock occur a 

 apparently in irregular veins, the junctions of which with the fine-grained dolerite are 

 " remarkably abrupt." 



A. H. Phillips 5 noted in the trap of Rocky Hill, New Jersey, a coarse-grained 

 phase whose relation to the other phases is not mentioned, except that it is regarded as 

 occupying a central position in the intrusion. The pyroxene crystals " constantly 

 increase in length as we pass in from the border " of the intrusion; in the very coarsest 

 varieties of the rock they measure " often an inch and occasionally 2 inches in length." 

 A schistose arrangement was sometimes seen, and slight curvature of the long axis was 

 observed, but this is evidently attributed by the author to the pressure which gave 

 rise to the schisosity. 



The internal characters of the pyroxenes are similar to those described for the 

 Adelie Land rock, but the basal striation is attributed to polysynthetic twinning parallel 

 to (001). Undulatory extinction was observed of a zonal nature. 



The " plumose diabase " occurring as a phase of the trap sheet of Holyoke, 

 Massachusetts 6 , appears to be a particularly coarse-grained variant of the basalt of 

 which the sheet is composed. The pyroxenes of this rock, up to 4 inches in length, 

 are curved, twinned on the pinacoid, and basally striated ; and in addition the vertical 

 sections show notched or serrated edges due to " the development of unit faces." These 

 coarse-grained patches occur as lenticular masses or sctdieren in the trap. 



The similarity on many points of the Adelie Land rock with those just mentioned 

 is very clear, and it seems as though the characteristics emphasised, especially those of 

 the pyroxene, are peculiar to certain pegmatitic phases of the dolerites, so that the 

 rock No. 733 may with some degree of confidence be assigned to this category. 



4. CHEMICAL COMPOSITION AND RELATIONSHIPS. 



To indicate the chemical characters of the Horn Bluff dolerite a chemical analysis, 

 for which I am indebted to Mr. G. D. Osborne, B.Sc., and Miss M. L. Graham, B.A., 



1 Q.J.G.S., vol. 1, 1894, pp. 311-336. 2 Elsden, op. cit. * Annual Report of State Geologist of New Jersey, 1907. 

 Teall, op. cit., p. 643. '" A.J.S., 4th series, vol. viii, 1899, pp. 267-285. " Emerson, Bull. Geol. Soc. Amer., 

 vol. 1904, pp. 91-130. 



