Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/82

 by which it appears to brave the external war of the elements which surround it, it is no less distinguished from the stratified trap than it is from the neighbouring Red mountains, whose rapidly decomposing summits covered with fragments have long lost their characteristic forms, hastening with daily and visible waste to the level of the plain below. It is equally distinguished by its general aspect, being commonly separated by marked features from the terraced trap with which it is associated, and no less so from the Red hills, with which, however associated in almost a common group, it forms no union. In mineral composition it also differs widely from these, hornblende forming its predominant and characteristic ingredient, as felspar does that of the latter. From the terraced trap it differs less in composition, since hornblende is the predominant ingredient of both, but it never like this contains amygdaloids and beds of tufo, neither does it entangle coal, sandstone or ironstone, or exhibit the beautiful zeolites so abundant in the terraced trap.

If there were any theory of the formation of the rocks of this class on which we could rely, it would not be useless to examine how far their distinctness of character and appearance was consonant to it, or how far they differed. Although my chief object has been to describe the geological features of the island, yet I may suggest that such an independence of character and form seems to imply at least that some independence in circumstances, as well as in time, has accompanied the formation of these several portions of a rock associated in so many other essential parts of their leading characters. But the time for discussing questions of this nature does not appear yet to have arrived, and conjectures may be left to the ingenuity of those to whom they afford a source of gratification.

With this rock I terminate the history of the principal rocks of Sky.