G. .W. Tyrrell—Petrography of South Georgia. 487 
1. Lpidiorite (Meta-dolerite).—This is a medium-grained greenish 
rock occurring in Gold Harbour. In thin section it shows large 
irregular plates of a pale hornblende in ophitic relations with numerous 
small laths of labradorite. There is a quantity of interstitial, 
probably secondary, quartz and leucoxenic material. The rock is 
obviously derived from an ophitic dolerite, but the original augite 
has been changed to hornblende. The latter mineral has only a slight 
pleochroism from almost colourless to a pale apple-green. The rock 
has suffered some crushing as evidenced by lines of granulitized 
material, and by the occasional fracture of the felspar laths. It is 
probably to be correlated with the decomposed ophitic dolerite 
described from Mr. Ferguson’s original collection,’ and is no doubt 
also a sill in the Lower Division of the Cumberland Bay Series. 
2. Ophitie Dolerite and Basalt.—These rocks occur in Larsen and 
Slosarezyk Bays, and also along the coast between Slosarezyk Bay 
and Cape Disappointment. In hand specimens they are dark, fine- 
grained rocks, mottled black and white, and with an ophitic texture 
obvious upon inspection. In thin section the rocks are seen to consist 
of a plexus of plagioclase laths in ophitic relations with irregular 
plates of a pale purplish augite. There is also a quantity of green 
alteration products derived from the pyroxene, and subordinate iron- 
ores. The felspar gives symmetrical extinction angles in the neigh- 
bourhood of 30 degrees, and is therefore an acid. labradorite (Ab, 
An,). Zoning is not conspicuous, and when it occurs there is only 
a slight change in the composition of the felspar, which becomes 
andesine (Ab, Ang) on the margins of the crystals. The augite 
forms small anhedral plates of the same order of magnitude as the 
‘felspar. Consequently it does not often entirely enclose the felspar 
laths, but is merely indented by their terminations. It is almost 
colourless, but frequently becomes a pale brown or purplish-brown 
towards the edges of the crystals and slightly pleochroic. While the 
‘bulk of the mineral is fresh it is changing along the margins to 
a pleochroic variety of chlorite. Some sections of this substance 
show a good cleavage, and strong pleochroism from bluish-green to 
pale yellow. The mineral gives a nearly uniaxial figure, and is 
probably to be referred to the penninite variety of chlorite. It has 
suffered much solution and redeposition, and has consequently 
migrated into the fissures and cavities of the other minerals, especially 
into the felspars, which are sometimes well reticulated with thin 
films of chlorite along their cleavage planes. The iron-ore is rather 
sparse, irregular, and skeletal in form, and is titaniferous, since it 
gives rise to a grey leucoxenic alteration product. It is usually 
enclosed in the augite, especially near the margins of the crystals. 
There is a notable absence of apatite in these rocks. 
The quantitative mineral composition of these dolerites is easily 
obtainable by the Rosiwal method, and is as follows: Plagioclase 
(Ab, An,), 42°0; augite, 36:7; chlorite, 13:5; iron-ore (titaniferous 
magnetite), 5°6; pyrite, 2°2 per cent. As is usual with ophitic 
dolerites the rock is mafelsic (i.e. the felsic constituents are roughly 
1 Tyrrell, op. cit., p. 830. 
