THE OLD RED SANDSTONE. 25 
of Plutonic action in the locality, into a rounded hill of mod- 
erate altitude, but of huge base. The upheaving power con- 
tinued to operate —the gneiss and mica-slate gave way a-top 
— and out of this lower dome there arose a higher dome of 
granite, which, in an after and terminating period of the inter 
nal activity, gave way in turn to yet a third and last dome of 
porphyry. Now, had the elevating forces ceased to operate 
just ere the gneiss and mica-slate had given way, we would 
have known nothing of the interior nucleus of granite — had 
they ceased just ere the granite had given way, we wouid 
have known nothing of the yet deeper nucleus of por- 
phyry ; and yet the granite and the porphyry would as. 
suredly have been there. Nor could any application of 
the measuring rule to the side of the hill have ascertained 
the thickness of its outer covering — the gneiss and the mica 
schist. ‘The geologists of the school of Werner used to illus- 
trate what we may term the anatomy of the earth, as seen 
through the spectacles of their system, by an onion and its 
coats : they represented the globe as a central nucleus, encir- 
cled by concentric coverings, each covering constituting a geo 
logical formation. The onion, through the introduction of a bet- 
ter school, has become obsolete as an illustration; but to restore 
it again, though for another purpose, we have merely to cut it 
through the middle, and turn downwards the planes formed 
by the knife. It then represents, with its coats, hills such as we 
describe — hills such as Ben Nevis, ere the granite had per- 
forated the gneiss, or the porphyry broken through the granite. 
If it be thus unsafe, however, to calculate on the depth of 
deposits by the altitude of hills, it is quite as unsafe for the 
geologist, who has studied a formation in one district, to set 
himself to criticise the calculations of a brother geologist by 
whom it has been studied in a different and widely-separated 
3 
