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Chapter V. 
THE DOLERITES. 
The dolerites, intrusive in the Beacon sandstone and the granite, are remarkably 
uniform in appearance and in microscopic characters, from Depot Nunatak at the head 
of the Ferrar Glacier to the Kukri Hills near its mouth. 
The rock (662) from Depot Nunatak is fairly typical of all the specimens (see 
Plate X, Fig. 1). It is a mottled gray-brown medium-grained dolerite, showing no 
porphyritic crystals. Under the microscope it is seen to be made up mainly of 
colourless augite, partly in long prismatic crystals, and partly in irregular sub-ophitic 
plates, and plagioclastic felspar (labradorite chiefly) in stout prisms and lath-shaped 
crystals. Grains of magnetite and ilmenite are very sparingly distributed. 
A characteristic feature of most of these dolerites is the 
presence, in patches and in the interstices of the augite and 
felspar, of more acid material, showing quartz in radiating 
(spherulitic) and micropegmatitic intergrowth with felspar. 
In the section of specimen (662) quartz is seen to have 
crystallised round prisms of felspar, from the end of which 
springs a micropegmatitic intergrowth (see Fig. 72). 
In this respect, as well as in their general characters, 
these dolerites bear a striking resemblance to the so-called 
“ augite-diorites with micropegmatite,” which are intrusive 
in the gneisses and pyroxene-granulites of Southern India 
(Madras Presidency), and have been described by Dr. T. H. 
Holland.* In both cases the coarseness of grain of the 
micropegmatite varies directly with the texture of the rock, and thus one of the 
principal arguments used by Dr. Holland in favour of the primary origin of the micro- 
pegmatite is applicable also to these Antarctic rocks. In some specimens, however, 
( e.g ., 692 from Dry Valleys) this more acid and (in the case of this specimen) finer- 
grained felsitic material occurs in such distinct patches (up to 2-3 mm. in diameter), 
with interspaces nearly free from it and consisting simply of the felspar-augite aggregate, 
as to suggest an intermingling of two rocks (granophyre and gabbro), such as occurs so 
commonly in Skye.f It is remarkable that in this rock the magnetite (in rod-like 
skeleton-crystals) is mainly confined to the more acid patches. Some of the specimens 
of dolerite found (but not in situ ) at Depot Nunatak are gabbro-like in coarseness of 
grain, and in these the micropegmatite is also coarse-grained, and constitutes practically 
the ground-mass of the rock. One of these gabbro-like rocks (632) contains olivine 
intergrown with the augite, which in this case is of the purplish titaniferous variety. 
* Holland, Quart. Journ. Geol. Soc., 1897, vol. liii, p. 405. 
t Harker, The Tertiary Igneous Hocks of Skye. Mem. Geol. Surv. of the United Kingdom, 1904, p. 169. 
Fig. 72. — Micbopegmatite in 
Dolebite (662) fbom Depot 
Nunatak. (Magnification, 100 
diam.) 
