Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/278

 as well as of the detached pieces and small points, since they are absolutely identical in composition with the smaller reticulations.

With respect to the limestone, it is subject to a variation of aspect where it approximates to the granite which it is interesting and important to notice. The colour of the great limestone mass is lead blue of various intensity, and its texture is almost universally large grained and highly crystalline. It is true that in some few instances it preserves this aspect in the situations described, but, in the greater number of cases, particularly in those where it is penetrated by the smaller veins and where the beds have undergone an elongation or a contortion, the crystalline texture disappears. It then assumes an appearance exactly resembling that of hornstone or compact felspar, having a smooth texture, with a thin-edged fracture intermediate between the splintery and flat conchoidal. I may as well add here that the schist is often of an argillaceous, but highly indurated, character, and also that it often passes into hornblende slate, and this (it is worthy of notice) the more frequently as it approximates nearer to the granite.

So much has been said with respect to the origin of granite veins, and on the mode by which their intrusion into the schistose rocks which generally accompany them has been effected, that it is superfluous to repeat it, since nothing new can be offered on the subject. By whatever mode it has been produced, it is obvious that the same explanation will apply to the case of the limestone which is interstratified with the schist, as far at least as the veins are concerned, although the fact itself be a new one. But how has the alternation of laminæ of granite with those of limestone been effected?

It is necessary here to anticipate that which will be fully described, and, I trust, proved hereafter in a more proper place, the