Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/318

 would be thrown aside as a bad specimen of such a rock. I would not be understood to mean that the substances in question are really the fragments of shells, whatever general resemblance they may possess; but if considered merely as crystallizations they are sufficiently singular to be worthy of notice, since nothing analogous to them occurs in the different schistose rocks which I have examined in Scotland.

One other rock is yet deserving of notice which, although not precisely situated in Ben Gloe itself, is found among the beds which appertain to this great mass of quartz rock. It is visible in Glen Fernat.

This is a mass of porphyry intermixed with a mass of quartz in such a manner that it is impossible to ascertain precisely the relation which either of them bears to the surrounding rocks. There is however little reason to doubt that the quartz forms a vein, but I suspect that the porphyry also is disposed in a similar manner, and that the appearance in question is the result of the interference of two veins. In either case the porphyry as a mineral specimen offers an aspect I believe as new as it is difficult to explain. Its basis is the usual indurated claystone, or compact felspar, or hornstone if that term be preferred, which is the most common basis of the porphyries that occur among these strata. It contains distinct crystals of felspar, but together with these, fragments of quartz are also found in it. These are most obviously fragments not crystals. They are irregular, of different sizes from that of a pea to that of an egg, and their angles are sharp. In addition to that, where the porphyry and quartz masses are in contact larger fragments are to be observed mutually connected both with the porphyry and with the quartz. Porphyry has generally been considered as a crystallized rock, yet here it offers the mixed structure of a crystallized