THE 



EOLOGICAL MAGAZINE. 



m. XXX.— DECEMBER, 1866. 



I. — On the Metamorphio Origin of Certain Granitoid Eocks 

 AND Granites in the Southern Uplands of Scotland. • 



By James Geikie, Geological Survey of Great Britain. 



THE special proofs of metamorphism wliicb. it is the object of this 

 paper to point out, are rather of a geological than chemical nature. 

 They consist, in short, of such evidence as may be readily gathered 

 in the field, and although, to render them complete, they ought per- 

 haps, to be followed up by analyses of the rocks, yet it is believed 

 that the phenomena to be described tell a plain enough story. 



The rocks referred to below are Diorite, Minette, and Granite, all 

 of which, with one exception, are admitted by most geologists to 

 have generally had an igneous origin, — that is to say, they have not 

 only been in a state of fusion, but have also at various periods forced 

 themselves among pre-existing strata. For example, diorite and 

 minette are met with abundantly throughout Scotland as eruptive 

 rocks — the former seeming to prefer strata of Carboniferous age (to 

 which, however, it is not confined) — the latter seldom travelling 

 beyond the range of Old Eed and Silurian regions. But the diorite 

 to which reference will presently be made is of a more granitoid and 

 less basic character than the usually finer-grained and often dull 

 earthy rock of the dykes which here and there intersect the Coal- 

 measures.' So far as its mineralogical nature is concerned, however, 

 the granitoid diorite alluded to, might be erujpti'm, for it is simply an 

 admixture of hornblende with white and pink felspar. Minette, or 

 quartzless granite, is a rock that sometimes forms intrusive dykes 

 through Lower Silurian greywackes, very beautiful examples of 

 which may be seen in Priesthill Bum, near the village of Inner- 

 leithan, on the river Tweed. The exclusively eruptive origin of 

 granite is advocated by several geologists ; but the proofs which are 

 thought to establish this theory have been called iu question, and 

 the metamorphic character of the rock strenuously upheld. In the 



^ Most of the trap dykes of the Scottish Oarboniferoiis strata, however, are not 

 hornblendic but augitic greenstones, or dolerites. 



VOL. III. — NO. XXX. 34 



