318 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. 



blende-schist by the contact of fused granite, it might be 

 expected that the same change would happen when interposed 

 between two beds of gneiss ; for the heat transmitted must 

 have been very great, in order to form the more distant bed 

 of gneiss. Is it, however, equally satisfactory, when the clay- 

 slate is found alternating with gneiss, as in the islands of 

 lona and of Isla; or when, as in the same isles, and in 

 various parts of Scotland, this slate gradually passes into and 

 alternates with hornblende-schist ? For if clay-slate, when 

 heated to a certain degree, becomes hornblende-schist, by 

 what laws of caloric could the intermediate beds of clay-slate 

 have escaped this conversion? This argument is also ap- 

 plicable to gneiss : thus, in Tirey, and in other Scottish 

 islands, the highly crystalline or granitic variety is found 

 interlaminated with that which is regularly schistose ; and not 

 only so, but the latter sometimes completely envelopes the 

 former, and this again encloses not only irregular veins and 

 layers, but also entirely insulated nodules or masses of perfect 

 granite. Turn again to the mica-slate and clay-slate of the 

 eastern part of Ireland, as described by Weaver, and there 

 too occur phenomena of the same import ; and in Cornwall, 

 the misnamed clay-slates not only envelope and are inter- 

 laminated with hornblende and other crystalline substances, 

 but also contain insulated masses of granite, which, in one 

 instance, at Herland Mines, have been discovered at a 

 distance of two or three miles from the main granite. It is 

 useless to multiply examples, for one is sufficient ; the fact 

 cannot be disputed, that rocks supposed to require an intense 

 heat for their production, such as would fuse granite, are 

 found enveloped in other rocks which could not have expe- 

 rienced such an elevated temperature. How are such capri- 

 cious operations of caloric to be accounted for ? The easiest 

 method would be to doubt the perfection of a theory which 

 has involved such a dilemma, or at least to reject the con- 

 dition of the primary slates as evidence in favour of an igneous 

 metamorphosis. 



