Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/429

 dip of the rocks of the island is from north to south, the Oolitic series is best seen at the north end, whence its different groups of strata slope northward, until they finally disappear under the volcanic series at the Bay of Laig. The following section shows the order of succession among these rocks —

Dolerites, basalts, &c. of the plateau.

(4) Clay, with ammonites, belemnites, &c, seen for a short space on the beach of the south side of Laig Bay.

(3) Estuarine shells and limestone with Cyclas, Cyprides, Ostrea, &c.

(2) Thick white and yellow sandstones, in some places abounding with fragmentary plants, and at other parts strongly calcareous, with numerous casts of Cyclas, &c.

(1) Estuarine shales and limestones, with fossils similar to those in No. 3, and with reptilian bones.

Base of series not seen.

From the general character of these beds, I am inclined to regard them as on the same horizon with the estuary beds of Loch Staffin, and with those which in Raasay come in between the Lias and the Tertiary volcanic rocks — that is, as belonging to the Lower Oolites. But this subject will require further consideration when the fossils have been determined.

D. Volcanic Series.

The Oolitic strata of Eigg are overlain by a cake of igneous rocks, which, though it caps the northern cliffs, dips southward with the underlying strata, until it reaches the sea at Laig Bay on the west side, and at Kildonan on the east. North of a line drawn between these two places the igneous capping has been reduced by denudation to a mere narrow strip, forming the tableland of Beinn Bhuidh ; south of the same line, it covers the whole breadth of the island. Yet, although the general inclination of the igneous and aqueous rocks is in the same direction, a careful survey of them shows that the former lie unconformably upon the latter. At the south side of the Bay of Laig, the basalt-rocks rest upon the clays of group No. 1. As the former are inclined at a slightly less angle than the latter, they soon creep over their edges, so as to lie upon the shales and and limestones No. 2. These continue as far as Dunan Thalasgair ; but there the basalts, after slowly creeping over their denuded edges for nearly two miles, overlap them, so as to come upon the massive sandstones of group No. 2. The apparent conformity, therefore, of the volcanic rocks with the Oolitic strata of the inner Hebrides, which has led to the belief that the volcanic phenomena were of Oolitic age, is in reality deceptive. I shall on other occasions have to point out the varied horizons on which these volcanic masses rest.

The igneous rocks of Eigg may be most conveniently described under three heads : —

1. The Basalt-plateau, marking the oldest eruptions.

2. Intrusive bosses, sheets, dykes, and veins.

3. The pitchstone coulees of the Scur, a relic of the last eruptions.