46 Mac Culloch on Fitchslone. 



converted into enamel. These singular varieties occur in the 

 hill of Glamich in Sky, as well as in Arran ; and, it is interest- 

 ing to remark further, the connexion which they possess with 

 pearl-stone. 



In tracing the progress of common pitch-stone to pearl-stone^ 

 it will sometimes be seen that a spheroidal concretionary, but 

 indistinct, structure exists in some of these, which, by a gra- 

 dually increasing distinctness of the concretions, causes them 

 to pass into the latter substance. In others it may be observed, 

 that the enamel grains just mentioned, are the centres of a 

 spheroidal concretionary tendency ; and that such varieties are 

 in fact imperfect pearl-stones. In the pearl-stones of decided 

 character, it will also be found that the grains or spherules 

 contain a central enamel, and these varieties have been called 

 pearl-stone porphyry. It is, moreover, interesting to observe, 

 that in other cases the enamel and the investing pitch-stone 

 spherule seem to have entered into combination ; the result 

 being a pearl-stone of a distinct and peculiar character, but in 

 which the enamel grains may still in some part or other be 

 discovered. 



I may, lastly, remark, while on the subject of the porphyritic 

 structure, that in some rare instances, of which Arran affords an 

 example, the feldspar crystals consist of successive concentric 

 prisms, a layer of pitch-stone being interposed between each, 

 so as to produce a compound crystal. 



There is no stronger proof of resemblance between pitch-stone 

 and the trap rocks, than the transition from that substance to 

 basalt, a change which is far from being uncommon. The 

 rock which forms the Scuir of Egg already mentioned, holds a 

 place pretty nearly intermediate between the two. Nor are 

 similar transitions rare in basaltic veins in many other cases ; 

 particularly where one such vein traverses another of a larger 

 size. In many places, and remarkably at Loch Scavig in Sky» 

 the basaltic ramifying veins that traverse the hypersthene rock, 

 become gradually finer in texture as they divide, until the 

 minute filaments are converted into perfect pitch-stone. 



In the same island, a vein of basalt of a considerable size is 



