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successive layers of limestone, we find these becoming much 
thicker towards the east, where the delta was advancing most 
rapidly; while on the west, where clearer-water conditions obtained 
throughout the whole period, the drifted materials become finer 
and finer as they are traced from east to west, until they nearly 
disappear altogether, and nothing but limestone is left. 
Geologists recognize in the Lower Carbouiferous Rocks of the 
North of England three, or four, subdivisions. These are, counting 
from below upward, (1) the Upper Old Red, (2) the Lower 
Limestone Shales, (3) the Mountain Limestone proper, which is 
represented by the Calciferous Sandstone Series in Scotland ; and 
(4) the Yoredale Rocks, which as yet have not been referred to 
here. 
The Yoredale Rocks, which succeed the Mountain Limestone, 
consist of a series of alternations of beds of limestone, with beds 
of sandstone, shale, chert, and coal. These are the rocks that 
constitute the greater part of the so-called Mountain Limestone in 
North Cumberland, in Scotland, and especially along the Pennine 
Escarpment. Most of the dales of North-West Yorkshire have 
been carved out of the same rocks. 
Their geological history forms a continuation of that described 
under the Mountain Limestone. ‘The chief difference between the 
two lies in the fact that the individual limestones forming the most 
conspicuous members of the Yoredale Rocks, are even more 
persistent than the beds of the Mountain Limestone proper. On 
this account each bed of limestone has received a definite name, 
which is recognized by miners and others over many hundreds of 
square miles, from North Yorkshire through Westmorland, Durham, 
Cumberland, and Northumberland. These limestones, being so 
persistent, can be traced over a very large area; and we now know 
that, north of an east and west line through, say, Penrith, most of 
the Lower Carboniferous Limestones appertain, not to the Mountain 
Limestone properly so called, but to the higher subdivision known 
as the Yoredale Rocks.* The individual beds of Yoredale Lime- 
stone thin slightly towards the north, but the rate of thinning is so 
* The Yoredale age of the Carboniferous Limestones of North Cumberland 
was determined by the writer nearly twenty years ago, and has been repeatedly 
communicated without reserve to various fellow-workers ; one of whom, in 
publishing the information, has virtually claimed the discovery as his own. 
