VI. Erratics.

Boulders do not occur very plentifully in Orkney; but we felt convinced, from an examination of those we met with, that they must have been mainly distributed during the primary glaciation. In Westra blocks of granite and quartzite arc found on the slopes of Cleat hill; and rounded stones and boulders of red sandstone from Eda occur in the southern district as well as along the western shores.

In the north of Sanda, at Saville, a remarkable boulder of gneiss is met with, which has been described by previous observers. It measures × 6 × 2½ feet above ground, but its base is buried underneath the surface. Professor Heddle, who has made a minute examination of this boulder, states that it docs not appear to be a British rock. He gives the following description of it in a recent number of the 'Mineralogical Journal'^*:—" It consists in greatest amount of white finely striated oligoclase, the crystals of which are penetrated by fine filaments of actinolite, glassy quartz in much smaller amount, dark green finely foliated lustrous hornblende in well-marked crystals, very little of a pale-green mica, a minute amount of a pale-brown mineral, which may, but does not appear to be sphene, and a speck or two apparently of thorite. The mass also contains a single crystal of pale-green apatite four or five inches in length by over an inch in width, and this apatite contains imbedded cryptolite."

He states that the only Scotch rock resembling the Saville boulder which he is aware of is to be found in Sutherlandshire; but it has orthoclase as its felspar, and does not contain apatite. Should this boulder really prove to be of Scandinavian origin, its presence page 660 has an important bearing on the question! of the extension of the ice in the North Sea. Some smaller blocks of gneissose rocks occur in the neighbourhood. A few boulders of conglomeratic sandstones occur in Eda, which may be purely local.

On the Mainland blocks of white and reddish-grey sandstone are strewn on the hill-slopes north of Finstown and on the moory ground south of Maes Howe, which have been derived from the north-west shore of Scapa Flow; and so also along the west coast, between Brak Ness and Inganess, north of Hoy Sound, boulders of granite and gneiss are met with on the flagstone area to the west, of the axis of crystalline rocks.