1866.] J. GEIKIE CAERICK, AYESHIEE. 521 



spar-porphyry, which here assumes the character of an overlying 

 trap, with a most uneven bottom. At its southern boundary it 

 presents some remarkable appearances, losing its usual amorphous 

 nature and becoming largely spheroidal. The spheroids have a 

 reddish hard-glazed exterior, and show innumerable spherical cavi- 

 ties like mere pin -punctures, but occasionally larger. The spheroidal 

 masses measure from two or three to five or six feet in diameter. At 

 their junction with the greywacke, the portions visible are encased 

 in shells about half an inch in thickness of hard fine-grained 

 reddish greywacke, much jointed, and sometimes crystalline like 

 felstone. To what extent the porphyry spheroids are enclosed in 

 these peculiar greywacke shells or cases was not ascertained. When 

 two spheroids impinge, the " case " in which each is enveloped 

 appears as if it had been sucked in between the rounded masses 

 of felspar-porphyry. The shells may be separated from the rock ; 

 and their inner surfaces are then found to be coated with porphy- 

 ritic felspar crystals ; but these crystals nowhere penetrate beyond 

 the surface, to which they appear to be merely adherent. 



In other places, where enveloping shells or cases are wanting, a 

 distinct passage can be traced from the porphyry into the greywacke. 

 The porphyritic crystals become gradually smaller, until at last they 

 disappear, and thus leave a red compact or finely crystalline fel- 

 stone. This rock, again, shades ofi" into hard reddish greywacke. 



The passage from amygdaloid into felspar-porphyry has been, 

 given above ; the latter, indeed, is most commonly associated with 

 the former. 



jRecapitulation. — Before attempting to describe the other forms of 

 metamorphism displayed in this district, I may recapitulate some of 

 the facts advanced, and state the inferences which seem to be fairly 

 deducible from them. 



An examination of the less altered rocks of the metamorphic 

 region will show that the strata are of variable composition, some 

 portions being more highly alkaline than others. The large amount 

 of calcareous, and especially of magnesian matter is indeed very 

 striking. This alkaline character of the strata may be considered 

 one of the causes of that intense metamorphism to which many of 

 the beds have been subjected; for those which contain the largest 

 admixture of alkaline matter appear to have been peculiarly sus- 

 ceptible of change. The green magnesian and calcareous greywackes 

 are often studded with vesicular and amygdaloidal areas ; on the 

 other hand the less alkaline deposits are frequently merely har- 

 dened, without assuming any pseudo-igneous structure. A broad 

 general view of the whole series of strata thus gives alternating masses 

 of more or less distinctly crystalline and altered rocks ; but when 

 viewed in detail the various metamorphic rocks are generally found 

 to be confusedly mingled together, interlacing and blending in the 

 most irregular manner. 



The little patches of unaltered greywacke which occur here and 

 there in midst of metamorphic strata are interesting, as they seem 

 to throw some light upon the process by which the felstones, amyg- 



2n2 



