Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/434

 volcanic rocks of the plateau. It consists of a red gravelly matrix of dolerite debris, in which are imbedded angular and subangular fragments of various igneous rocks, sometimes a foot and a half long. Again, at the south end of the island, opposite the rock called Dubh Sgeir (Black Skerry), the dolerites contain a breccia which swells out rapidly from a few inches to 6 or 8 feet in thickness ; it is a rough nodular bed, varying in colour from a dirty green to a dull red, and consisting of rude angular and subangular pieces of various dolerites, but more particularly of that on which it lies, imbedded in, or wrapped round by, a greenish more or less crystalline paste, veined with calc-spar.

2. Intrusive Bosses, Sheets, Dykes, and Veins.

The Oolitic rocks, as well as the basalt-plateau which lies upon them, are pierced by many intrusive masses of igneous rock. These are all crystalline rocks, no example of any intrusive fragmental mass, such as the agglomerate of necks, having yet been noticed. While in the interbedded series the order of superposition furnishes us at the same time with the relative age of the volcanic beds, among the intrusive rocks we have no certain guide to relative antiquity, save the obvious examples where one rock cuts through another. Nor is it easy to discover any means of ascertaining how far the intrusive masses were coeval with, posterior, or anterior to those of the plateau. The dykes, indeed, must be newer than the interbedded rocks already described ; for they are found cutting through even the highest of the sheets of the plateau, as well as the intrusive sheets near or at the base. There is reason to think that the pitch- stone-veins are yet more recent. But without attempting any chronological arrangement, let me here describe the intrusive rocks of Eigg, in accordance with the nomenclature above proposed, as capable of classification after the character of the mould into which they have been intruded.

a. Amorphous Masses or Bosses.

Only three amorphous intrusive masses were observed by me in Eigg ; but they possess considerable interest, inasmuch as they serve to throw some light upon the age of similar masses in Skye. They consist of felstone (that is, a supersilicated felspar rock, with a little free quartz), and thus stand out strongly marked from the surrounding basic basalt-rocks. The largest and most characteristic forms a range of bold cliff, from 150 to 200 feet high, at the extreme north end of the island. It appears to have risen approximately along the bedding of the Oolitic strata, and thus to form of itself a large rude bed. It consists of a pale grey quartziferous porphyry, traversed by horizontal and oblique veins of basalt. It is quite columnar in places ; and as the sea has here and there hollowed out caves at the base of the cliff, the roofs of these recesses expose the truncated ends of the columns. This rock closely resembles some of the finer-grained parts of the quartziferous porphyries of Skye and