4 
Classification of Rochs 
man. The granite cuts through, sends veins into, and 
alters both systems; it is therefore more modern than the 
carboniferous system, probably of the age of the rathe 
toclte lieyende , or lowest part of the new red sandstone. 
We have from these discoveries the following remarkable 
results. West Devon and Cornwall, instead of primary 
rocks, contain nothing older than the old red sandstone; 
their granite, instead of being the most ancient, is the 
most modern of their rocks. The old red sandstone, or 
Devonian system, consists, on the north of the Bristol 
Channel, of red sandstones, marls, and conglomerates; on 
the south, of yellow and variegated slates and limestones. 
The carboniferous system, in like manner, changes its 
usual character for that of a mass of roofing slates, with a 
few’ beds of dark limestones. 
When now we find small portions of the earth exhi¬ 
biting such great changes in the mineial tructure ofthe 
same formations, and such great errors respecting the 
age of igneous rocks, in a country 90 well known as 
England, it behoves us to be especially cautious in our 
classification and nomenclature both of aqueous and 
igneous formations in countries widely distant from the 
localities whence our names and arrangements are drawn. 
The European geologist, in approaching distant countries, 
must loose his hold of much of his previously acquired 
knowledge ; dismiss from his mind all the arbitrary and 
minute divisions to which lie has been hitherto accus¬ 
tomed, and hold them at bay until he see whether or not 
they be applicable to the things he is now studying. He 
must at once fall back on the general principles on which 
all geological classification ought to be founded; and, 
guided solely by these, separate the rocks he meets with 
into those portions and divisions only which naturally 
belong to them. AY hen each large portion of the globe 
shall have been examined, and its constituent portions 
I 
