Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/192

 as a whole, is not less symmetrical than that of the faces of Staffa, while at the same time they far exceed it in grandeur as well as in absolute magnitude. Their height reaches from 200 to 300 feet and upwards, a dimension, however large, not sufficient to overpower the due proportion which should exist between the aggregate structure and the parts of which it is composed, since the magnitude of the columns is proportioned to their height, and the total effect therefore similar to that of Staffs, where the proportions are so nicely adapted for beauty.

With respect to the composition of this variety of trap, there is necessarily some uncertainty, since the great extent of it, as well as the inaccessible nature of most parts, renders it utterly impossible to examine it throughout. We also know that the various members of this family are often found irregularly intermixed, so that to have ascertained the composition of one portion of a mass, gives us no assurance that we have made ourselves acquainted with that of the whole. Yet I am inclined to think that the greater part will be found to consist of a substance analogous to greenstone, in which augit occupies the place of hornblende, a rock of great frequency in Scotland, and often, perhaps generally hitherto confounded with common greenstone, unless in a few such remarkable cases as that of Rum, where the substances are too distinct to admit of mistake. It may be called augit rock, without introducing any confusion into mineralogical nomenclature.

For the sake of topography I must here mention a small mass of trap, lying on a part of the coast of Sleat not easily visited, and omitted in the original paper. It occupies a projecting point south of Talivil, where its place has been marked in the amended map. It covers a space of about a mile in extent, lying over the red sandstone. It is rudely columnar and slightly porphyritic, and