362 DAKYNS : LOWER CARBONIFEROUS ROCKS IN YORKSHIRE. 
diately overlaid by the Millstone Grit : this begins at Lower- 
head merely as a thin cherty top to the limestone ; but the 
chert gradually develops into a series of cherty beds, shales, and 
sandstones, known as the Black and Red Beds in Swaledale. 
Still further north the cherty beds change into a set of water- 
bearing sandstones, grits, and shales, known as the Coal Sills, 
overlaid by a thin but persistent bed of limestone, known as 
the Little Limestone. 
4. Owing to the deterioration of the lowest Millstone Grit in Walden 
and Coverdale and on the tiank of Penhill, it is somewhat 
uncertain what line should be taken further north as the Mill- 
stone Grit base, so as to keep to the same horizon. But in 
ni}'' opinion the best line (at least the most certain line) to 
take is the top of the cherty series and its equivalent, the 
Little Limestone. I am aware that lithologically this would 
not be a good line in some parts of Northumberland ; but then 
we should at all events keep to one and the same horizon. 
5. It is important to notice that the silicious grits and ganister-like 
beds that occur in the Millstone Grit series above the Kinder- 
scout grits become more pronounced northwards, so that at 
length they become regular ganister measures, similar to those 
that occur in the lower part of the Coal Measures. 
