Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/192

 much iron and standing in a situation approaching to perpendicular in many instances, might in time become natural magnets, with I the south pole uppermost and the north pole lowermost.

This however, I have never found to be the case; but I remarked a complete polarity in a rounded block of greenstone that was lying rather deep in the ground on the top of Donald-hill in the county of Londonderry: the needle of a pocket compass that had been laid upon it went half way round and stood there permanently, the north pole of the needle pointing to the south and the south pole to the north.

The pillars of Fairhead, and the adjacent summit of Fairhead, are composed of greenstone containing augite.

Basalt passes into greenstone in the most imperceptible manner: in the first stage, the felspar exists under the form of short, scaly parts either white or greenish; the texture becomes more finely granular and crystalline; the fracture in the great is not into flat conchoidal pieces, but more usually into irregular and blunted fragments. This finely granular and almost compact greenstone is also more tough than common flœtz-trap.

3. Greenstone.

It is difficult to determine the precise geognostic relations of the basalt and greenstone in this district; but since the latter here occupies an extent comparatively small, it must be considered as a formation subordinate to the former; whether however they form distinct beds, or pass insensibly into each other by a gradual transition, is as yet undecided.

The greenstone columns of Fairhead and Cross hill are destitute