SKY. GEOLOGY. OVERLYING ROCKS. 375 



rather have defined this rock by its base than by its 

 structure or accidents ; but mineralogical language has 

 no means of distinguishing, not only the infinite difference, 

 but the perpetual variation of the bases which contain 

 these nodules of occasional minerals. It is sufficient to 

 say, that the base varies from the hardness of basalt to 

 nearly the softness of dry clay, and that the colours 

 are black, blueish, brown, dark purple, and grey of dif- 

 ferent tints, sometimes of a very pale tone. As these 

 varieties often occupy different strata, and are variously 

 intermixed with the solid kinds already described, the 

 strata, when viewed in the cliffs, often seem to possess 

 a diversity of composition which, when examined into, 

 proves fallacious. The nodules imbedded in these amyg- 

 daloids are very various, few of all the substances usually 

 met with in the trap rocks being wanting in some part 

 or other of Sky. The zeolites are the most conspicuous, 

 since most of the species are found occupying these cavi- 

 ties, and often in forms so important as to require a separate 

 consideration in the description of individual minerals. 

 Calcareous spar, chlorite, steatite, quartz, chert, chal- 

 cedony, and prehnite, occur in other varieties; among 

 which the two latter are most rare. At Talisker, mica 

 is to be observed in some of them, a substance among 

 the least common ; and in the vicinity of Scavig, epidote 

 is an ingredient, the rock resembling precisely the speci- 

 mens from Caer Caradoc. 



Many varieties of greenstone are found among the 

 strata in different parts of the island; but they are far 

 less common than the simpler rocks. In some instances 

 the crystallization of the hornblende is very perfect. 

 Basaltic porphyries occur also in different places, but, 

 like the greenstones, they are much inferior in quantity 

 to the uniformly basaltic substances. The felspar is 

 sometimes glassy, at other times opake, and the com- 

 pound occasionally forms beautiful specimens. Wacke 

 is I believe unknown in this island, although some of 



