532 GeiUe — Metamorphic Origin of Granite. 



Sueli are some of the more interesting appearances exliibited by 

 these altered sandstones of Old Eed age — appearances which we find 

 reproduced in certain true granites. 



Loioer Silurian straia, and associated Grey Granites of Southern 

 Scotland. — The grey granites of the Lower Silurian uplands of 

 Southern Scotland are associated with the usual grey and gTcyish 

 blue felspathic greywaekes, interbanded with occasional beds and 

 broad belts of shales and mudstones. In these greywaekes flakes 

 or galls of hardened clay or shale and mud are of common occurrence. 

 Those of Peeblesshire, for instance, and the corresponding rocks 

 ia Carriek and Galloway, furnish excellent examples. And so abun- 

 dant do the galls sometimes become, that they impart to the beds a 

 finely brecciaform aspect. There can be no dovibt that were the 

 strata of those regions attacked by metamorphism they would give 

 rise to rocks of dissimilar appearance. The felspathic greywaekes 

 would necessarily assume a crystalline structure more readily than 

 the less easily reducible shales. Hence we should expect to find that 

 any alteration affecting the strata en masse would be more marked in 

 the former than in the latter — in other words, that crystalline beds 

 would alternate with fine-grained compact and jaspideous rocks. 



A very characteristic granite of the district under review is that of 

 Loch Doon, Ayrshire. It is a ternary compoiind of the usiial kind 

 — grey or white felspar, white quartz, and black mica. In some 

 places it also contains hornblende. It is bounded by a set of hard 

 fissUe slaty shales, which, along the line of junction, are crumpled 

 and puckered, much indurated, and finely crystalline. Across the 

 strike of these beds, the granite does not extend, but it is otherwise 

 where the strata are highly felspathic, these last having apparently 

 offered no hindrance to its out-growth. The same phenomena have 

 been observed by my colleague, Mr. B. N". Peach, who has detected 

 several undescribed masses of granite to the north-east of Loch Doon. 

 These granites are developed in certain broad bands of vertical 

 felspathic greywaekes, flanked on either side by hard flinty shales, 

 which are finely crystalline along the line of junction with the 

 granite ; but this metamorphism quickly ceases as we recede from 

 the junction, at right angles to the strike. On the other hand, when 

 we leave the gTanite, and proceed along the direction of strike, we 



certain horizontal lines -which seem to coincide with the bedding, leaving intermediate 

 spaces of a finely crystalline greyish felstone. Flakes of dark shale, somewhat less 

 highly altered than those alluded to above, occur in these incipient schistoze portions of 

 the minette. But from this, it is not inferred that an amorphous crystalline state is 

 necessarily preceded by a streaky or schistoze condition of the minerals. With regard 

 to the peculiar arrangement of the minerals in gneissoze and schistoze rocks we have 

 still muchJx) learn. It is certain, however, that rocks such as diallagite, hypersthe- 

 iiite, diorite, syenite, and even granite itself can be developed directly from aqueous 

 rocks, without passing through an intermediate gneissic or schistoze state. This will 

 probably always be the case when the aqueous rocks acted upon are thick -bedded and 

 of an equally diffused composition. The exact conditions which gave rise to our 

 schistoze-, hornblende-, diallage-, and hypersthene-rocks, and to gneiss itself have yet to 

 be discovered, but it is not unlikely that thin bedding and variable composition of the 

 alternating layers may have been among the causes that determined a schistoze 

 arrangement of minerals. 



