408 SIMPSON : STRATA AND DEPOSITION OF THE MILLSTONE GRITS. 
the neighbourhood of Richmond to that of Leeds. A little to the 
north of Leeds the southern boundary of this easterly extension dips 
under the Coal Measures ; north from Richmond the grits extend in 
outliers, and again margin, on the west, the Durham and Northum- 
berland Coal-fields. 
It would be impossible, even in so general a manner as I pro- 
pose, to consider them otherwise than as they may be studied in our 
own more immediate neighbourhood. 
It is in our own neighbourhood, that is in Lancashire and 
South Yorkshire, that the Millstone Grit attains its greatest develop- 
ment ; with the associated shales the thickness varies from about 
1,500 to nearly 5,000 feet, and from this central tract the beds thin 
away in every direction ; most rapidly towards the north, north-west, 
south-west, and south-east ; less rapidly towards the south and east. 
In the extreme north, that is in Cumberland and Northumber- 
land, the grits are not only thin but fine in texture, and more like, if 
not actually finer, than our Lower Coal Measure sandstones. In 
considering this succession of sandstones and shales, which we call 
the Grit Series in our West Yorkshire district, with survey maps 
perhaps for guidance, one is confused by the divisions and nomencla- 
ture generally adopted. 
Where the series was first mapped, that is in Derbyshire, the 
beds conform to a broad general classification into four grits, with 
their dividing shales. These sandstones were therefore named down- 
wards, the First, Second, Third, and Fourth Grits, the first being 
further known as the " Rough Rock," and the fourth as the 
" Kinderscout," so named from the precipitous ridge on the west 
side of the Peak of Derbyshire. 
Following the series northwards, however, whilst there was no 
difficulty in recognising the Rough Rock or the Kinderscout, the 
Second and Third Grits grew increasingly confusing and complicated, 
and difficult to correlate with the Derbyshire members. 
In our Halifax district we get at least four important sandstones, 
with their shales, constituting a considerable thickness, which we 
attempt to correlate with the Third Grit ; whilst the one we call the 
second has become almost unrecognisable as a separate sandstone, 
being rarely parted from the Rough Rock. 
