About two years ago, Capt. Moore, of H.M.S. ‘Penguin,’ brought home to this country and forwarded to the Admiralty block of a white rock obtained from the cliffs at Fanny Bay, Port Darwin, in the Northern Territory of the Colony of South Australia. The specimen was forwarded by Capt. W. J. Wharton, F.R.S., the Hydrographer to the Admiralty, to Sir Archibald Geikie, For. Sec. R. S., Director-General of the Geological Survey, who, with Capt. Wharton's consent, kindly allowed me to make a microscopical examination of it, and the results seem of sufficient interest to communicate to the Society. The rock in question is of a dull-white or yellowish-white tint, in places stained reddish with ferruginous material; it has an earthy aspect, like that of our Lower White Chalk, but it is somewhat harder than chalk, though it can be scratched with the thumb-nail. There are no signs of stratification, and it appears as a fine-grained, homogeneous material. Unlike chalk, however, it gives no reaction either in cold or heated hydrochloric acid. When thoroughly dry it readily breaks up into flakes with uneven surfaces. Though somewhat soft, thin microscopical sections of it can be prepared without much difficulty, and these show a fairly transparent groundmass containing numerous very minute granules and subangular mineral-fragments ranging up to .075 mm in breadth. The groundmass itself is quite negative in polarized light, and appears to be made up of amorphous silica, but the minute grains and angular particles with which it is filled readily