Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/286

 is 1531.7 feet above the level of the road at Ballinteer; and the Three Rock mountain is 1247.9 feet above the same place, the elevation of which is considerable. The highest point of Howth is 567 feet above high-water mark.

Account of Minerals, &c.

I. Vesuvian. — (Idocrase, Hally). This substance was observed by Mr. Stephens in specimens found by me at Kilranelagh, where it occurs in irregular crystalline masses, in a rock composed of common garnet of a reddish-brown colour, of quartz for the most part greenish, apparently from the admixture of a lamellar fossil of that colour, and a small quantity of felspar. The crystalline form of the garnet is here often very distinct, but in the specimens hitherto found, that of the Vesuvian is not well exhibited, although some indistinct prisms are to be observed. In general, its particles assume a scapiform aggregation, sometimes approaching to stellular, a form which I have not observed in specimens of this substance from other places; but its fusibility, lustre, colour, and other characters leave no doubt as to its nature.

The blocks of this compound at Kilranelagh were not in their natural place, but their size, their great weight and angular form, render it probable that they were not far removed from it. Garnet rock is described as occurring in beds in primitive mountains, and the country at Kilranelagh is of that description.

It is remarkable, that a compound much resembling that which I have described, occurs also in the County of Donegal, from whence specimens now in the cabinet of the Dublin Society, and that of Dublin College (No. 30.), were obtained. The garnet and vesuvian in these specimens, are scarcely to be distinguished from those of