144 G. T. PRIOR. 



rocks present striking similarities with the dolerites which 

 were found by H. T. Ferrar, the Geologist to the Discovery 

 Antarctic Expedition, occurring in numerous dykes and sills 

 in the " Beacon Sandstone " of South Victoria Land.^ They 

 are also very similar to the dolerites ("quartz-diabases") in- 

 trusive in the Newark (Triassic) sandstones and shales of New 

 Jersey.^ 



Structural Variations. — In structure the specimens 

 show great variations, in all probability depending mainly 

 upon the distance from the sedimentary rocks into which the 

 dolerite dyke or sill is intrusive. In some of the more coarse- 

 grained varieties the augites show under the microscope 

 irregular prismatic sections, generally twinned and exhibiting 

 the well-known " herring-bone " structure due to fine striations 

 parallel to the basal plane. In most of the coarse-grained 

 specimens, however, strongly marked ophitic structure is 

 naturally the most common. Beautiful examples of this 

 structure are afforded by the olivine - dolerites mentioned 

 above, and also by olivine-f ree dolerites intrusive in sandstone 

 at Newcastle and at Ashwell on the Umvoti River, Natal. 



From well-marked ophitic structure all gradations are passed, 

 by diminution in the size of the augite-j^lates, to a granular 

 intersertal structure, in which the augite occurs in small 

 grains in the interstices of the felspar laths. Of examples of 

 sub-ophitic type maybe mentioned dolerites intrusive in coal- 

 bearing rocks on Umhlali beach, and rocks intrusive in Ecca 

 shales on Ixopo Creek, Natal. In many of these rocks the 

 sub-ophitic plates of augite exhibit a curious undulose extinc- 

 tion, a black bar sweeping round as the nicols are rotated. 



In this place may be mentioned the fine-grained " amygda- 

 loidal pipe-basalts " of Anderson (see 'First Report,' p. 61), 

 which occur to the north of the Lower Umfolosi Magistracy 

 in Zululand (125), for, except that they contain a few amyg- 



' See National Antarctic Expedition, 1901-4 : vol. i, ' Geoloc^y," pp. 49, 

 136. 



^ J. V. Lewis, ' Geol. Surv. of New Jersey,' Annual Report for 1907, 

 1908, p. 97. 



