228 



PETROLOGY OF ROCK COLLECTIONS 



In the slide the bulk of the constituents is found to be composed of coarse lath- 

 shaped interlacing plagioclases ; the maximum symmetrical extinction angle on the 

 albite twins is about 35 degrees, it is therefore a lahradorite. The bronzite is fresh. 

 There is present a very little apatite. 



THE DOLEEITES 



The occurrence of extensive dolerite pipes and sheets intrusive in the Beacon Sand- 

 stone formation over a large area in the Ferrar Glacier region has been ably described by 

 H. T. Ferrar.* The latter, arguing from physiographic forms, further suggested the 

 extension of the area affected by these intrusions far to the northward and southward. 

 Our collections show the truth of this surmise even to a still wider extent. Fortunately, 

 as shown by Dr. Prior, | certain characters of this rock are quite distinctive, and serve 

 as reliable criteria in allocating erratic specimens to that particular intrusion. A 

 typical specimen of this dolerite is described by Dr. Prior as being of a mottled grey- 

 brown colour, medium -grained and showing no porphyritic crystals. Under the 

 microscope it is seen to be composed mainly of colourless augite — partly in long 

 prismatic crystals, and partly in irregular sub-ophitic plates — and plagioclase felspar 

 (labradorite chiefly) in stout prisms and lath-shaped crystals. Grains of magnetite and 

 ilmenite are very sparingly distributed. 



The characteristic feature which serves as a criterion for distinction is the presence, 

 in the interstices of the rock, of more acid material showing quartz in micropegmatite 

 intergrowth with felspar. 



This micropegmatite is sometimes abundant, in other cases it is almost absent. 



The texture of these rocks varies in different outcrops, and is sometimes gabbroic. 

 In the same way slight alterations in the mineral constituents are to be noted, for 

 instance, in the amount of the residual micropegmatite ; at other times olivine makes 

 its appearance and the augite may be of the purplish titaniferous variety. 



An analysis made and quoted by Dr. Prior of the dolerite from Knob Head, Ferrar 

 Glacier, is as follows : 



Total 



92 10 



Chemico-mineralogical Classification : Class 3, Order 5, Eang 4, Sub-rang 3 (Auvergnose) . 



Dolerite erratics of this class occur throughout the whole range of our collections. 



Three specimens are represented in the Beardmore Glacier material. These are of 

 the normal type with the typical micropegmatite patches. The ferromagnesian mineral 

 is augite, which is undergoing uralitisation, in some cases more advanced than in 

 others. 



Loc. cit., p. 19. 



t Lor- cit., p. 136. 



