438 G. W. Tyrrell—Petrography of South Georgia. 
equal to the mafic), and the felspar and augite are developed in 
approximately equal quantity. 
There is no information as to the field occurrence of these rocks, 
but their microscopical characters.suggest that they probably occur as 
small intrusive bodies, such as dykes and sills. ‘his impression is 
strengthened by the presence in the collection of an ophitic basalt 
which is almost certainly the contact-facies of one of the dolerites. 
The mineral composition of this rock is identical with that of the 
dolerites, but it has a much finer grain. There is also a distinct 
increase in the proportions of augite and leucoxenic iron-ore, whilst 
the rock is impregnated with pyrite. The augmented proportions of 
the mafic constituents, and the finer grain, supports the view that 
this rock: is the contact-facies of the dolerite. It is recorded 
as having been obtained from the coast between Slosarezyk Bay 
and Cape Disappointment. Doleritic rocks so much decomposed 
as to merit no further description were collected in Larsen and 
Slosarczyk Bays. 
3. Alaskite.—This name was given by Spurr to an Alaskan granitic 
rock consisting essentially of quartz and alkali-felspars, and practically 
devoid of ferromagnesian constituents... The rock may also contain 
a little soda-lime felspar and muscovite. A rock of this type occurs 
in Cooper Island, off the extreme south-eastern end of South Georgia. 
In the hand-specimen it is a granular white rock with a very few 
dark specks representing decomposed biotite. The thin section shows 
that the rock consists of a granular mixture of quartz and orthoclase. 
Some of the latter shows mottling between crossed nicols, due to an 
admixture of the albite molecule, but most of the orthoclase is 
perfectly uniform and may be regarded as comparatively pure. 
Occasionally there is a granophyric intergrowth between orthoclase 
and quartz. ‘The only other constituents are oligoclase in very small 
amount, and a very few flakes of decomposed biotite. 
The quantitative mineral composition of the rock, as estimated by 
the Rosiwal method, is as follows: Quartz, 31°9; orthoclase, 65°6; 
oligoclase (Ab, An,), 2°3; biotite, -2. The rock is therefore 
practically holofelsic. It corresponds well with the mineral com- 
position (by norm) of the Alaskan types. In three analyses of the 
latter? the quartz ranges from 40:1 to 32-9, the total felspar from 
559 to 63°8, and ferromagnesian constituents from 1:1 to 2:4 per 
cent. The South Georgian occurrence is therefore slightly less 
quartzose than the typical Alaskan rocks. 
4. Quartz-felsites.—These are compact white, grey, or yellowish 
rocks in which a minutely nodular or spotted appearance may be 
frequently seen. These rocks are abundant in Slosarezyk, Larsen, 
and Drygalski Bays at the south-eastern extremity of South Georgia. 
They may be divided into two groups according to whether they are 
spherulitic or not. 
Microscopically these rocks consist of small phenocrysts of quartz 
and felspars in a microcrystalline to cryptocrystalline quartzose 
1 Twentieth Annual Report of United States Geological Survey, pt. vil. 
2 Iddings, Igneows Rocks, vol. ii, p. 453, 1913. 
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