GEOLOGICAL DISTRIBUTION. 99 



free from the matrix ; and it is from decayed material of this description that Mr. 

 J. Smith, Mr. John Young, and Mr. J. Bennie have obtained the remarkable 

 spicules of Hyalostelia, Tholiasterella, and other genera (Pis. VI, VII, VIII), and 

 Mr. J. Wright those of Spiractinella (PI. VIII). Not infrequently also the spicules 

 on the outer portions of the Sponge-beds weather out naturally so that their forms 

 can be ascertained. 



In some of the Sponge-beds the anchoring- and body-spicules of hexactinellid 

 Sponges appear to predominate ; other beds are mainly made up of very minute 

 acerate and cylindrical spicules of monactinellid Sponges, whilst in others, larger 

 acerates and occasional trifid spicules indicate that tetractinellid Sponges 

 contributed largely to the contents of the bed. 



The Carboniferous Sponge-beds referred to above are principally developed 

 in the upper portion of the Yoredale series, between the summit of the Carboniferous 

 Limestone proper and the base of the Millstone-grit. Bands and nodules of chert 

 also occur in the Carboniferous Limestone, and some which I have examined are 

 filled with spicules, like those in the series above. 



Yorkshire. — In the higher districts of Swaledale, Wensleydale, and Arkendale, 

 in the north-west of Yorkshire, 1 the chert and siliceous rocks, composed of Sponge 

 remains, are well-known and persistent portions of the Yoredale series, and from 

 the fact that in common with the associated limestone and other rocks they are 

 traversed in places by veins of lead-ore which have been largely worked, the various 

 beds have been recognised by distinctive names. Thus the upper beds are 

 known as the " Red-Beds Chert," and below these and separated from them and 

 from each other by intervening beds of limestone, shale, and sandstone, are the 

 "Black-Beds Chert," the "Main Chert," and the " Undersett Chert." The 

 thickness of these different chert-beds varies in different sections. 



The " Undersett Chert" exposed in the bed of the Swale, both below and 

 above Keld, appears to be from 15 to 20 feet (4*5 to 6 m.) in thickness. 

 It is here a dark, compact, brittle chert. In a railway- cutting near Leyburn,' 

 in Wensleydale, the " Undersett " is 10 feet (3 m.) in thickness. The chert here 

 is greyish, distinctly banded, and filled with spicules. The " Main Chert " is well 

 shown at Arkendale, on the south side of the valley; in the upper portion 

 of Gunnerside Gill, in Swaledale ; east of Leyburn in Wensleydale, as well as 

 other places in the district. It more frequently consists of a compact, dark grey 

 mottled, siliceous rock, rather than a true chert. The beds are continuous, with 

 a total thickness of about 18 feet (5-4 m.). The " Black Beds " and " Red 

 Beds Chert " are not so well exposed as the beds beneath them, but according to 



1 For much of the information respecting the distribution and the best exposures of the cherty 

 rocks in this area I am indebted to Mr. J. G. Goodchild, F.G-.S., of the Geological Survey. 



