422 On the ROCKS in the 



I have had an opportunity of examining many veins of 

 greenftone ', but I know of none more interefting in a geolo- 

 gical point of view than this. 



I think it can fcarcely be doubted, that the fame effort 

 which feparated the included portion of fandftone, cleft the 

 correfponding ftripe of greenftone from the great bed. This, 

 as well as the gradation of grain, everywhere obfervable in beds 

 and veins of trap, are remarks, in my opinion, of confiderable 

 value to the Huttonian hypothecs. On a former occalion, 

 when I had the honour of fubmitting fome remarks on the 

 north of Ireland to the Society, I took an opportunity of 

 dwelling particularly on the laft . circum fiance. Like the 

 charring of coal, when that fubftance is found in contact 

 with whin, as has been ably remarked by Profeflbr Play- 

 fair, " few fads in the hiftory of foflils fo directly af- 

 " fimilates the operations of the mineral regions with thofe 

 " which take place on the furface of the earth *." This gra- 

 dation of texture has a ftrong analogy to many accidental facts 

 obfervable in furnaces, of glafshoufes and the like, and ftill more 

 fo to thofe experiments made exprefsly for the purpofe of af- 

 certaining the effects of flow cooling, by Sir James Hall and 

 others. One additional argument for the igneous origin of thefe 

 veins, has been added by the obfervations of Sir George Mac- 

 kenzie and his friends, in Iceland, in perfect correfpondence 

 with the above fact. He there found many veins of this fub- 

 ftance, coated on the ildes with a glafty covering, exactly fimi- 

 lar to melted greenftone, when rapidly cooled. 



I should expect the fame circumftance would be met with in 

 veins of porphyry and granite ; but I have not been able to ex- 

 tend 



- Illuftrations of the Huttonian Theory, § 68. 



