SPENCER : THE YOREDALE AXD MILLSTONE GRIT ROCKS. 389 
down the left side of the valley to this place, a distance of five 
or six miles. 
From Sowerby Bridge down to North Dean the Third Grits, 
which appear to decrease in thickness as they go eastwards, 
gradually sink below the surface, and finally the Rough Rock, the 
uppermost member of the Millstone Grit rocks, sinks under the 
Lower Coal Measures at Halifax and Elland. 
Sections I. and II. will show that in this district there 
is no break or natural dividing line between the Yoredale strata, 
and the Millstone Grits, nor between the latter and the Lower 
Coal Measures. This is fully borne out by evidence derived from 
the study of the fossil shells found in these beds. The whole series, 
of Yoredale, Millstone Grits, and the Halifax Coal strata of our 
districts appear to be estuarine deposits, but in some places, as in 
the neighbourhood of Harrogate, Ripon, Fountains Abbey, Tees- 
dale, itc, the corresponding beds are largely of marine origin, 
and contain a large number of Mountain Limestone forms. We 
allude to this fact to remind the student that whilst our local 
beds of sandstone and grit and shales were being deposited in a 
large estuar}', some 40 or 50 miles to the north-east contempo- 
raneous deposits were being formed in the sea, consisting mainly of 
limestone and calcareous shales and thin grits, and containing a 
similar suite of marine fossils to those of our Millstone Grit shales, 
but with a greater proportion of deep-sea forms. During the 
frequent oscillations of the levels of the land and sea-bed, our 
Millstone Grit area must have been frequently in communication 
with the sea, whose inhabitants, migrating into our area, left their 
remains in the fossiliferous beds of the Millstone Grit shales. 
The Millstone Grits are divided from one another by thick 
beds of shale; each of these contains one or more beds of fossil 
shells and fish-remains. All the .species found in these beds also 
occur in the Yoredale strata ; the chief difference between the 
two suites seems to be that the species become fewer and fewer 
in number as we ascend in the series of beds. The number of 
specimens of shells in some of the beds is as large, if not larger 
