52Q PEOCEEDIKOS or THE GEOLOGICAL SOCIETY. [JuHG 6, 



well-marked bedding ; but occasional dips may be observed. Their 

 general character is that of a shattered, close-grained, finely crystal- 

 line, sometimes compact rock, becoming coarsely crystalline and 

 even porphyritic in places. Passages from hardened granular grey- 

 wacke into semicrystalline rock, and from that into the close-grained 

 felstone, are of common occurrence. Not infrequently areas of 

 granular Avacke may be obtained completely encircled by felstone, 

 both being evidently parts of the same bed; and, on the other 

 hand, portions of finely crystalline felstone make their appearance 

 in the midst of comparatively unaltered wackes. These are cer- 

 tainly not dykes, nor intrusive masses, but are merely meta- 

 morphosed parts of the beds among which they occur; for they 

 distinctly shade ofi* into the semicrystalline and granular matter 

 which encloses them. They seem in this way to be analogous to 

 those vesicular areas so characteristic of some basic greywackes. 

 The greywacke beds which have been partially converted into com- 

 pact felstones are most usually fine-grained rocks with little or no 

 calcareous or magnesian matter ; and this insufficient supply of alka- 

 line matter probably accounts for the absence of amygdaloidal cavities. 

 "Whenever the strata begin to get alkaline, the altered crystalline 

 areas become amygdaloidal. 



When the strata have originally consisted of alternations of very 

 highly basic with less alkaline beds, the resulting metamorphic masses 

 exhibit a great variety of rocks. Thus on the shore at Lendalfoot 

 the sea-stacks and skerries show confused alternations of felstone, 

 felspar-porphyry, hardened granular greywacke, altered hmestone, 

 hyperite, diorite, diallage-rock, serpentine, and occasional brcccii- 

 form beds ; but although there is much confusion in detail, still, when 

 viewed on the large scale, the direction of the masses agrees with the 

 strike of the rocks of the district. The fine-grained felstones here- 

 abouts are remarkable chiefly as showing how felspathic rocks may 

 in places become diorites. In this metamorphic district no rigid 

 line can be drawn between the two, either by mineralogist or geolo- 

 gist. Near Lendalfoot the felstones are often thickly studded with 

 blotchy crystals of white felspar, which, being much harder than the 

 matrix, stand out in relief on weathered surfaces. These crystals 

 are aggregated in a very irregular manner. They are of small size 

 where they are most abundant ; but in the more sparsely porphy- 

 ritic areas of the felstone they are usually larger, being sometimes 

 an inch in length. Occasionally the beds show more or less distinct 

 crystals of hornblende, which increase in number until, by their 

 abundance, they impart a dark hue to the rock. This, then, we 

 should call diorite. But for fuller details I must again refer to a 

 forthcoming memoir of the Geological Survey. 



III. DioKiTic Bocks. 



The dioritic strata do not occupy so large an area as the felspathic 

 rocks; they are also of more interrupted occurrence, frequently 

 appearing as small lenticular layers interstratified with serpentines. 



