THE DOLER1TES OF KIN(J KEORGE LAND AND ADELIE LAND BROWNE. 247 



An almost universal characteristic of the enstatite-augite is its undulose extinc- 

 tion, which is sometimes zonal in character, but generally quite irregular. This is a 

 feature which has often been observed in the pyroxenes of the quartz-dolerites, but has not 

 yet been satisfactorily explained. Benson remarks on something analogous in connec" 

 tion with the rhombic pyroxenes in the dolerites of South Victoria Land 1 , comparing 

 it with the similar appearance presented by anorthoclase, and ascribing it tentatively 

 to submicroscopio twinning. It will be remembered that Brogger and Harker 2 believe 

 anorthoclase to be of the nature of a cryptoperthite ; and it seems possible that this 

 peculiar form of augite, containing a large proportion of the enstatite molecule, may 

 be a similar cryptoperthitic intergrowth of ihombic and monoclinic pyroxene, a view 

 developed by Elsden 3 . 



It may be that the ruonoclinic pyroxene at the temperature of crystallization 

 was able to hold in solid solution a certain proportion of enstatite, more than it could 

 retain at a lower temperature. On cooling, therefore, the excess of enstatite was 

 expelled and took up a cryptoperthitic relation towards the augite, and the volume 

 change, if any, involved in this process possibly set up a state of strain and produced the 

 undulose extinction. Alternatively if the mineral is still a true homogeneous mix- 

 crystal, a state of internal strain may perhaps have been set up owing to a tendency 

 on cooling for the mineral to change its system of crystallization. 



The habit of the pyroxene is peculiar ; there is a suppression of the prism faces 

 and a flattening of the crystal parallel to the (100) pinacoid, so that cross-sections are 

 rectangles with the length about four times the breadth. The prisms are perhaps 

 rather more elongated than usual parallel to the c axis, and this characteristic is very 

 marked in specimen No. 733, in the description of which it will be more particularly 

 referred to. 



The mesostasis which is present in all three of the rocks is usually of such fine 

 grain as to render the exact determination of its constituents a matter of much difficulty. 

 Under low magnifications it appears colourless but slightly clouded, and sometimes 

 almost isolates the plagioclase laths projecting into it. Viewed with higher magni- 

 fications it takes in most places the form of long slender rods of plagioclase arranged 

 in different patterns on a background of another felspar with the optical characters of 

 orthoclase. Tin- rods may be aggregated in groups, each consisting of three or four 

 ])ar;illel rods, crossing each other at various angles; or sheaf-like and plumose aggre- 

 gates, occasionally sprouting from the end of a plagioclase may suggest the effect 

 of frost on a window-pane ; or again, the rods may form a confused interlacing network. 

 These phenomena are similar to those described by Osaun for the Tasmanian dolerite 4 

 mid n\- Benson for that of Sm.th Yi( toria Land. 



1 Report of British Antarctic Expedition, 1907-9, (ieology (vol. ii), page 15 1. 

 1 Harker : Natural History of the Igneoua Roclu, page 246. 

 The St. David'* Head Rock Seriea. Q.J.O.8., vol. Ixiv, 1908, pag. 

 0ann: Central, fur Mill., 1907, pp. 701-11. 



