Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/425

 more than 1000 feet. Some of the noblest hills of the Inner Hebrides are but solitary outliers left standing amid the ruin of the great sheets of solid rock of which they once formed a part. Ben More, in Mull, though more than 3000 feet high, is only a magnificent fragment of the huge pile of volcanic material which formerly swept over what are now the deep glens and fjords of Mull. The long lines of imposing cliff with which the basalt plateaux front the Atlantic all through these islands, from the Fair Head of Antrim to the far headlands of Skye, tell everywhere the same tale of vast and continuous denudation. Great, therefore, as the area is over which these rocks are now to be traced, it covers but a small part of its original extent.

These prefatory remarks may suffice to show the general nature of the subject of which I propose to treat, and I shall now proceed to describe in some detail a district in which some of the phenomena are typically displayed. The area which I have selected for this purpose is the island of Eigg, partly on account of its simplicity of structure, and partly because it presents to us a more striking picture of the vast duration of the Tertiary volcanic period in Britain than any other space of like size with which I am acquainted. My observations are the result of a survey made by me of the island in the year 1864. In this excursion I was accompanied by my friend and former colleague Professor Young, of the University of Glasgow, who devoted himself to the palaeontology of the island. It was our original intention to combine our observations in a joint memoir. Circumstances having occurred, however, to delay the proper examination of the fossils, it has been judged expedient to publish, in the mean time, my own observations on the volcanic geology of the island, leaving the oolitic strata and their fossils to form the subject of a future communication.

THE ISLAND OF EIGG*

A. Physical Features and Geological Structure.

In the chain of the Inner Hebrides, broken as it is in outline and varied in its types of scenery, there is no object more striking than this island. Though only about five miles long and from a mile and a half to three miles and a half broad, and nowhere reaching a height of so much as 1300 feet, this little island, from the singularity of one feature of its surface, forms a conspicuous and familiar landmark. Viewed in the simplest way, Eigg may be regarded as consisting of an isolated part of one of the great basaltic plateaux which, instead of forming a rolling tableland or a chain of hills with terraced sides, as in Antrim, Mull, and Skye, has been so tilted that, while it caps a lofty cliff about 1000 feet above the waves at the

kindly revised for me by my friend Mr. Alexander Nicolson, advocate, whose name will be a sufficient guarantee for their accuracy.
 * The spelling of the Gaelic names on the map and in this memoir has been

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