E. 8. Willbourn—Alteration of Granitic Rock. 441 
disturbing the texture of the replaced rock except in the most 
extreme instances. ; 
6. Augitite.—Vhe pebble of igneous rock in Professor Pirsson’s 
collection is a dark, fresh, vesicular lava, with phenocrysts of a 
shining black pyroxene. In thin section it shows numerous small 
_phenoerysts of purplish augite and brown ‘basaltic’ hornblende in 
a groundmass consisting mainly of augite microlites, with subordinate 
euhedral magnetite and a colourless glassy base. The augite pheno- 
erysts are usually purplish in colour, but frequently become green 
towards the interior of the crystals. They are zoned in colour bands, 
differing slightly in optical properties, and enclose magnetite crystals, 
especially near their margins. The hornblendes are also euhedral, 
but are indented by the augites wherever they come into contact. 
They have a strong pleochroism in shades of brown. The augite 
microlites of the groundmass are almost colourless, and are embedded 
in a clear glassy substance of refractive index lower than that of 
Canada balsam. Therock is entirely felspar-free, and may be regarded 
as an augitite with hornblende. It is quite fresh and of recent 
appearance. It is a type of lava which is usually associated with 
series of alkaline character. 
* Conelusions.—The study of this collection unfortunately throws 
very little new light on the moot question of the tectonic. relations 
of South Georgia. The igneous rocks described are probably part of 
Heim’s ‘altvulcanischer’ area of the south-east end of South Georgia,’ 
and belong with the calcic end of the igneous series rather than with 
the alkalic; but rocks of a distinctively Andean type are still to be 
discovered. The pebble of augitite found by R. W. Chappell and 
received through Professor Pirsson is certainly of alkalic type, but on 
account of its mode of occurrence and isolated character no weight . 
can be attached to it. Hence the question as to whether South 
Georgia belongs to Suess’ ‘‘ Southern Antilles”? and is situated on 
a great loop connecting the Patagonian Andes with the mountains 
of Graham Land, or whether it is a remnant of an old sunken 
continental land, still remains open so far as the petrographic 
evidence goes.” The members of the Weddell Sea party of Sir E. 
Shackleton’s Transantarctic Expedition spent some time on South 
Georgia on their outward journey, and it is possible that their 
geologists found opportunity to collect data sufficient to solve this 
problem. 
IiJ.—Pyevumatrotytic ALTERATION OF A VERY FINE-GRAINED GRANITIC 
Rock From Neer Sempitan, Frperatep Matay Starrs.’ 
By H. 8S. WILLBOURN, B.A., Assistant Geologist, F.M.S. 
BOUT one mile to the north of the railway station at Ayer 
ui Kuning, in the south of Negri Sembilan, one of the Federated 
Malay States bordering on Johore and Malacca, is a low but con- 
spicuous range of hills covered with long grass. The highest of the 
1 Fr. Heim, Zeitschr. Ges. Erdkunde, Berlin, April, 1913. 
? Tyrrell, Trans. Roy. Soc. Edinburgh, vol. 1, pt. iv, p. 832, 1915. 
* Communicated with the permission of the Government Geologist, F.M.S. 
