Geikie — Mefamorpkic Origin of Granite. 533 



find the felspafhic greywackls altered to a mucli greater distance. 

 And it is further remarkable, that the more muddy beds which are 

 interstratified with these greywack^s, are the first to resume their 

 original character. The appearances at the immediate jmiction are 

 still more instructive. Where the hard slaty shales impinge upon 

 the granite we have no difficulty in laying our finger upon the line 

 that separates the one rock from the other ; but at the point were the 

 granite and the felspathic greywackes come together, the union of 

 the two rocks is so intimate that we have usually no line of demar- 

 cation, but, on the contrary, a gradual passage. 



" Nests " of altered rock in the Qrey Granites of tJie Southern 

 Uplands. — Those who have wandered over granitic tracts, must often 

 have seen little nests ^ of dark fine grained or semi-crystalline and 

 baked-like rock, sealed up in the solid gTanitic matrix. Very often 

 they show distinct traces of lamination, and perhaps their most nsnal 

 character is that of exceedingly fine-grained mica schist, the dark 

 almost black shade being due to the abundance of the mica, They 

 are usually of very irregular shapes, and are by no means confined 

 to those portions of the rock that abut upon the outlying bedded or 

 aqueous strata, but are scattered indiscriminately throughout the 

 granite. The granite and the rock of a nest are firmly knit together, 

 so that when a suitable edge has been obtained, a smart tap with the 

 hammer will fetch away a good specimen to show the junction. 

 This is commonly so marked that one may place a knife-edge upon 

 it, but I have sometimes (though rarely) met with "nests," the fine- 

 grained or compact rock of which seemed to pass by insensible 

 gradations, both of colour and texture, into the outside granite. As 

 far as my observation goes, the "nests " are as a rule, harder, tougher, 

 and less easily weathered than the granitic matrix, so that on exposed 

 surfaces the '• nests " usually stand out in relief. In some granites 

 and gTanitoid rocks, however, the reverse is the case, the "nests" 

 decomposing out and leaving behind them little pits, and irregular 

 hollows. When the granite and the contained nests are of the same 

 or nearly similar hardness, it sometimes happens that decomposition 

 has set in along the Ime that separates the one from the other, 

 affecting both equally. The short description already given of the 

 nests of altered shale in metamoi'iDhic minette and diorite has prepared 

 the reader for an explanation of the similar phenomena in granite.. 

 For if we must admit that the galls or nests of the former rocks 

 represent the small interrupted ribands and flakes of shale and mud,, 

 which occur so abundantly in the unaltered felspathic sandstones — 

 then, doubtless, we ought to allow a similar origin for the "nests" 

 so characteristic of the Ayrshire granites. These last either repre- 

 sent such little detached portions of shale as are of common occuiTcnce 

 in the Lower Silurian greywackes, or they may be the remnants of 

 thin bands or beds of shale that interleaved the original strata. 

 Those who deny the metamorphic origin of gTanite will probably 

 suggest that the nests of altered rock may have been caught up by this 



* They are well shown in many granites of the north of Scotland as well as in 

 those of the districts more immediately referred to. 



