78 Prof. Sedgwick on the Carboniferous Chain 



for roofing slate and flagstone, (e. g. Garsdale Hall pastures. High Pike in 

 Dent, and all round the upper precipices of Ingleborough, &c. &c.) They 

 are generally of a dark bluish grey colour, and are liable to decomposition. 

 Round Ingleborough they are however of a brownish grey colour, and of a 

 closer texture. Strong beds of grey sandstone occur in the lower part of this 

 division (c.) ; which sometimes, though very rarely, become of so coarse a 

 texture as to resemble millstone grit. 



Two thin bands of bright coal have been found (at Rawthay Gill and Ingle- 

 borough), one in the lower {a.) and the other in the upper division (c.) of this 

 group: and between Brough and Stainmoor (to the north of the line of section) 

 it contains a bed of coal, which has been worked to a considerable extent. 



In some of the hills near the head of Wensley Dale (e. g-. Stag's Fell near 

 Hawes, &c.) the shale-beds almost disappear ; and the group passes into a 

 complex deposit of hard, grey, micaceous gritstone, some parts of which afford 

 a good building-stone, and other more fissile parts, a material for flagstone 

 and roofing slate. 



9. Four-fathom Limestone. Notwithstanding the name by which it is de- 

 signated in the mining districts of the North of England, this limestone group 

 is of much more variable thickness than any of the three preceding (Nos. 3. 5. 

 7 .): but I believe that it in no instance entirely disappears. In some places it 

 is not more than ten or twelve feet in thickness ; in others it is much more ex- 

 panded, and is not less than thirty or forty feet thick. Changes ranging within 

 these limits are several times repeated, as may be proved by tracing these beds 

 through the mountains at the head of Dent and Garsdale. Its average thick- 

 ness in these regions is perhaps less than that which is indicated by its name. 

 The bottom of the series, as usual, is impure, and is deposited in irregular, 

 grey beds with traces of fossils. The middle portion is much more regularly 

 bedded, and contains innumerable fossils, especially encrinital stems. Indeed, 

 where the middle and upper portions of this group are largely developed, we 

 find that large encrinital stems form the greater part of the substance of the 

 successive beds; and where the same portions are ill developed, the stems are 

 fewer in number and less in size* — a fact, which shows the large share these 

 extraordinary fossils had in the formation of the several beds. 



At the top of this group we here and there find cherty, siliceous beds, with 

 casts of encrinital stems (like the well-known Derbyshire screw-stones) alter- 

 nating with bands of calcareous shale, containing innumerable, flattened Pro- 

 ductae. These beds, though unimportant in themselves, offer us a valuable 



* In the hills south of Dent the variety called bird's-eye marble abounds in this group. — See 

 above, Note -f- to page 75. 



