Geikie — Metamorphic Origin of Granite. 531 



variable composition, which, is sometimes a qiiartzless syenite, some- 

 times minette or mica-trap.^ Where this rock is typically developed 

 it shows a granitoid texture, and is, moreover, marked by special 

 phenomena, the meaning of which can be traced, and as these last 

 are frequently met with in granite, they seem to throw light on the 

 obscure history of that highly crystalline rock. 



The unaltered portions of the strata consist of reddish-, pinkish-, 

 and greenish-grey felspathic sandstones, coarse and fine-grained. 

 They contain little or no free silica, but often show a considerable 

 admixture of mica. The finer-grained beds are fissile or flaggy, 

 the surface of the flags being often coated with a pelKcle of pale 

 greenish clay, while here and there, both in the fine and coarser- 

 grained beds, the rocks show streaks of a similar green colour, 

 but generally of a darker hue. In man^^ places the sandstones are 

 marked with thin, short, lenticular bands or ribands, and with abun- 

 dant little galls or irregular blotches and flakes of brown, greenish, 

 and dark red clay or mudstone. 



The first indication of change presented by the strata consists 

 simply of hardening — the fracture becoming gradually more splintery 

 and conehoidal, as the induration increases. At the same time, the 

 texture begins to lose its dull sandy character, and to assume instead 

 a somewhat sparkling, compact, and baked appearance. Every 

 gradation from semi-crystalline to crystalline succeeds — in some 

 portions the rock being fine-grained, and the component minerals 

 indistinguishable — in other places the beds becoming changed into 

 distinctly crystalline and well-marked varieties of diorite and minette. 

 The little layers and flakes of clay mentioned above, have also 

 undergone change, but are still recognisable, even in the most 

 highly metamorphic portions of the strata, — here as hard splintery 

 Lydian stone and porcelanite, there as dark compact or fine-grained 

 crystalline rock, which is sometimes hornblendic, sometimes mica- 

 ceous, — diiferences that are no doubt due to the original chemical 

 nature of the separate " nests" and lenticular layers. For the ultimate 

 character of a metamorphic rock must always depend upon the com- 

 position of the stratum acted upon. In the coarser-grained minette,'^ 

 or quartzless granite, the included "nests " or galls reach the maximum 

 of their metamorphism. There they are changed into a hard, dark, 

 finely crystalline rock, in which minute points of mica could be detected. 

 It is somewhat noteworthy that, in this highly altered condition, the 

 "nests" seldom occur as lenticular layers, but most usually as 

 amorphous nodules and flakes.^ 



1 The best exposures of these metamorphic rocts occur on Tincorn Hill and Black- 

 side ; the unaltered strata are well seen in a few quarries in the same neighbour- 

 hood, and fine sections are also obtained in the tributaries of the Gower Water, a 

 stream that joins the Irvine above the village of Darvel. 



- It ought to be mentioned that here and there this minette contains a few crystals 

 of hornblende. 



2 It is impossible not to be impressed with the exact resemblance of these altered 

 fragments to the similar "nests" of metamorphic rock in granite, which are described 

 in the sequel. One is also interested by observing how the minette sometimes takes on 

 a faintly " schistoze" arrangement of its minerals. In a few places dark mica lies iu 



