422 On the ROCKS in the 
I nave 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 icarcely 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 hypothefis. On a former occafion, 
when I had the honour of fubmitting fome remarks on the 
north of Ireland to the Society, [ took an opportunity of 
dwelling particularly on the laft.circumftance. Like the 
charring of coal, when that fubftance is found in contac 
with whin, as has been ably remarked by Profeflor Piay- 
Farr, “ few facts in the hiftory of foffils fo dire@tly 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 Hay and 
others. One additional argument for the igneous origin of thefe 
veins, has been added by the obfervations of Sir GEorcE 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 fides with a glafly covering, exactly fimi- 
lar to melted greenftone, when rapidly cooled. 
I sHouLp expect the fame circumftance would be met with in 
veins of porphyry and granite; but I have not been able to ex- 
tend 
* Iluftrations of the Huttonian Theory, § 68. 
