I UK DOLERITES OF KING GEORGE LAND AND ADELIE LAND- UROWNE. 251 



It is instructive to compare the mineralogical constitutions of the three rocks 

 as given roughly by Rosiwal measurements : 



No. N... No. 



732B. 732A. 733. 



Felspar 41 54 



Pyroxene n .r? 27 



.Mesostasis ... II 11 38 



Iron Ore 1 2 



Altered Olivine (?) 3 



The rocks are placed in order of increasing average grainsize, and it will be noticed 



(1) That the ratio of light to dark constituents increases with grainsize, and 



(2) That the proportion of mesostasis has notably increased in the coarsest 

 phase. 



It should be remarked that accurate figures for the mesostasis in the other two 

 rocks were very difficult to obtain, and that a mental note was made, before results had 

 been calculated, that the mesostasis of No. 732B had probably been over-estimated, 

 and that of No. 732A under-estimated at the expense of felspar. Hence there is pro- 

 bably an actual progressive increase in the amount of mesostasis present with the 

 increase of grainsize. 



The texture of the mesostasis is likewise somewhat coarser in No. 733 than in 

 the other two rocks, a fact which is in harmony with the observations of Holland 1 , 

 Benson 2 , and others on the quartz-felspar mesostasis of quartz-dolerites. 



It is unfortunately not known from what part of the sill two of these specimens 

 originally came. That labelled No. 732B, from 6 feet above the base of the intrusion, 

 and exhibiting prismatic jointing, probably represents a marginal facies of the rock. 

 No. 732A, l>eing a bit coarser in grainsize, is probably from a more interior portion of 



the .si 11. 



It would be interesting to know the precise mode of occurrence of the relatively 

 coarse-grained type No. 733. This rock is strikingly similar in texture and constitution 

 to a coarse phase of the Tasmanian dolerite from the Domain, Hobart, similarity 

 i -.\ tending even to the curvature of the flattened and elongated pyroxene prisms. This 

 rock, according to Professor Sir Edgeworth David 8 , occurs as schlieren or pegmatitic 

 segregation veins in the normal fine-grained dolerite. In a specimen in the collection 

 of the Geological Museum of the University of Sydney, the pyroxene individuals 

 at times exceed an inch in length, and the mesostasis forms a large proportion of th e 

 rock. 



> Holland, Q.J.G.S.. vol. liii, 1807, p. 408. ' Bwuon, of. cit.. p. 155. ' Verbal communication. 



780-2 I'. 



