Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/272

262 probably not considerable. Nor indeed would a work which was intended for defence from within admit of a greater height of wall than five feet, or that over which a man might look, a height which is equal to that of the ancient British field works, if this may be determined from some of the perfect fragments which remain in Cornwall.

Of one of the most remarkable of these, I have given an account to the Antiquarian Society. It is the fort known by the name of Castle an Dinas, in the parish of Ludgvan, in that county. Here the altitude of the work is determined by the perfect finish of part of the remaining wall, which consists of well-fitted dry masonry, the strength and solidity of which show that it was not a temporary enclosure, but a sort of citadel, or work of permanent defence. The wall is here only live feet high, and from this I am inclined to conjecture that the vitrified forts did not exceed this height. Nor indeed are the accumulated ruins about them sufficient to give reason for suspecting that they ever were of a greater elevation. It is deserving of remark, that the vitrification of the outer work is not so complete as that of the inner ones.

Before examining the materials of which the wall is composed, it is necessary to mention the mineralogical nature of the rock on which it stands, and that of the immediate vicinity. The hill of Dun Mac Sniochain is formed of limestone, lying in schistus, similar to that which constitutes the neighbouring island of Lismore. The schistus and the limestone alternate, but the latter is the predominant rock. The hill itself is perfectly insulated in a great alluvial plain. To the west this plain is bounded by the mountains of Benediraloch, which descend abruptly into it, approaching at their nearest point within half a mile or less of the fort. These mountains are formed of the old rocks common to this country, granite, gneiss, mica-slate,