278 Proceedings of the Royal Physical Society. 
probability be distributed over more than the one period. 
(4) And finally, we have to take into account both the 
nature of the denuding agency and the relative destructi- 
bility of the strata removed. If we are sure upon all these 
points, we may make our calculations with more or less 
confidence. As an illustration of what is meant, it may be 
mentioned here that the Upper Cretaceous Rocks of Britain 
(and of Western Europe in general) lie upon the upturned 
and denuded edges of strata, extending from the summit of 
the Neocomians across the whole of the Jurassic Rocks, the 
New Red, the Carboniferous, the Devonian, and even on to 
rocks in Scotland which are almost certainly of Pre-Cambrian 
Age. Over all these strata, collectively many thousands 
of feet in thickness, the Upper Cretaceous Rocks lie with 
an evenness of base which is little short of marvellous. 
Yet we must be on our guard against supposing that the 
whole thickness of these strata was actually removed by 
denudation at this period. All that can confidently be 
stated is that the edges of the strata were planed to their 
present even form just before the Cretaceous strata were 
laid down. The total thickness of rock that can safely be 
regarded as having been removed by denudation which 
followed the Jurassic-Wealden Period and preceded the 
Neocomian and Cretaceous, is the aggregate thickness of the 
strata extending downward from the top of the Wealden to 
the base of the Upper New Red. We can easily satisfy our- 
selves that, traced westward along the southern parts of 
England, the Upper Cretaceous strata overstep the edges 
of the whole of the strata which are enclosed between the 
summit of the Wealden Rocks and the base of the Inferior 
Oolite. The Cretaceous Rocks of course le upon strata of 
Neozoic Age much older than that—but we can be 
quite sure that the thickness referred to did actually at one 
time exist, and was removed by denudation prior to the 
deposition of the strata that succeed. The total thick- 
1 T have long taught in my lectures that the peneplain of the Highland 
mountains, and of the mountain tops in the North of England, is the 
‘modified descendant” of this Pre-Cretaceous plain, upheaved, and 
re-exposed by subsequent denudation of the Cretaceous Rocks, 
