18 Proceedings of the Royal Physical Society. 
It will be observed that while the English subdivisions in 
the diagram show a sequence of strata actually traced through 
various districts, the Scottish strata do not similarly repre- 
sent such a natural classification. Care must therefore be 
taken, lest in an attempt after apparent uniformity of classi- 
fication of these latter strata the actual physico-geographical 
relations exhibited in the rocks themselves be set aside. 
A classification denoting the succession which actually may 
thus be traced in the coalfields of Scotland would be of far 
closer api^roximation to the truth than the one given above. 
Were such an inquiry entered into, it is highly probable 
that it will be found that the whole Scottish coal strata are 
not synchronous in geological age with the English series, 
but are parallel only with one or two of its lowest members. 
According to the diagram, the total thickness of the 
Lancashire strata is 14,200 feet, whilst that of the Lothians 
is only 6400 feet. The mountain limestone is one magni- 
ficent pile of calcareous matter in England, until it borders 
the confines of Yorkshire ; but the equivalent series in Scot- 
land, though containing one or two insignificant limestone 
beds, is in reality a succession of sandstones, coals, and shales. 
The Scottish analogues of the two intermediate English 
groups are confessedly imperfect or absent ; while even the 
true Scottish Coal Measures but poorly represent the magni- 
ficent English members of the series in thickness or position. 
Of course it will be remembered that even the smallest 
subdivision represents an enormous lapse of time ; a lapse 
of time, so great, indeed, as to allow of the most extensive 
changes in sea and land, and a persistence in such physical 
revolutions utterly alien to any of our notions founded on 
historic time. The cycles of change in a geologic seon are 
not at all to be measured even by the overturning of dynas- 
ties and kingdoms in our human epoch. Consequently the 
Scottish carboniferous strata may represent a period of phy- 
sical change very distinct from those chronicled by the 
English strata. The state of land and sea may have repeat- 
edly changed, — physical conditions requisite for the depo- 
sition of peculiar minerals may have been in action and 
ceased at different epochs of the great Carboniferous 8eon. 
