310 Dr. Mac CuLLocH 0// /Z>^ Geology of Glen Till. 



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 analo- 

 gous 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 

 Fernar. 



This is a mass of porphyry intermixed with a mass of quartz ill 

 such a manner that it is impossible to ascertain precisely the rela- 

 tion 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 inter- 

 ference 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 horn- 

 stone 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 crystal- 



