58 
DESCRIPTION OF THE ROCKS 
MILLSTONE GRIT SERIES. 
From what has been proved on the subject of the change of 
type of the lower scar limestones, and the Yoredale rocks, it follows 
that not only a simple homogeneous series of rocks may be rendered 
complex by the intervention of other rocks, so as to constitute a 
binary or triple succession, but also that the corresponding terms of the 
series are subject to great variation. Thus the main limestone has no ex- 
istence in the large tract between Coverdale and Wharfedale, the lime- 
stones above that of Hardrow scar vary much in regard to thickness and 
interval. Now of all the strata in the Yoredale series the limestones 
are by far the most continuous and permanent ; had we attempted 
to construct the diagram of corresponding geological age, in distant 
parts of Yorkshire, by a comparison of the sandstones and plates, the 
results would have been almost imaginary ; and lines of demarcation 
in the carboniferous series would have been worse than useless. 
The millstone grit series rests upon the Yoredale rocks ; but what 
shall we take as their common boundary ? 
Both of these groups of rocks consist of limestone, sandstones, 
shales, ironstones, and thin coal seams ; but while limestones abound 
in the lower group, sandstones predominate in the upper, and lime- 
stones become almost obliterated. If we were to proceed upon the 
principle of including all limestone beds with marine exuviae in the 
Yoredale series, we should have the following results. 1. A thickness 
of grits and shales to the extent of several hundred feet, with only 
a few yards of limestone, would be ranked with a series eminently 
calcareous. 2. The distinction for all parts south of the Craven fault 
and east of Wharfedale and Coverdale, becomes imaginary, or would 
divide the real millstone grit series into two parts. But, if instead 
of that arbitrary rule we look at the larger indications of nature, and 
make our classification fit to the physical aspect of the country and 
leading members of the groups, we shall include in the millstone grit 
