GEOLOGICAL DISTRIBUTION. oo 
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 (Pls. VI, VII, VIII), and 
Mr. J. Wright those of Spiractinella (Pl. 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. 
Yorksutre.—In the higher districts of Swaledale, Wensleydale, and Arkendale, 
in the north-west of Yorkshire,’ 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 (45 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. 
