100 BRITISH PALAEOZOIC SPONGES. 



Prof. Phillips 1 the thickness of these two upper series of beds in Arkendale and 

 Swaledale varies between 36 and 54 feet (10'8 — 16*2 m.). 



Lower down the Swale valley, and more particularly at Richmond (Yorkshire), 

 chert-beds of the same character are frequently exposed. In quarries above 

 Richmond there are numerous thin beds of chert and siliceous rock alternating 

 with crinoidal limestones, probably forming part of the " Red Beds." In the 

 section exposed, 34 feet in thickness, there are in all 9^ feet of the siliceous rocks 

 or Sponge-beds, which are filled with well-preserved spicules. 



Judging from the data given by Prof. Phillips, and from my own observation, 

 I should estimate that the Sponge-beds of Swaledale and Yoredale reach a total 

 thickness of between 70 and 90 feet (21—27 m.). 



In a quarry near Harrogate, in the so-called " Road-Stone," there are two beds 

 of chert with a thickness together of two feet. The spicules are well shown in 

 these beds. 



Lancashire. — Thin bands and patches of dark chert filled with spicules are 

 present in the Carboniferous Limestone exposed in the quarries near Clitheroe, 

 and thin bands of siliceous shale between the beds of limestone contain detached 

 spicules of Hyalostelia, Geodites, and Reniera. 



Noeth "Wales. — In Flintshire there is a continuous series of chert-beds, 

 which, as shown by the borings of the lead-mines, reach the extraordinary thickness 

 of 350 feet 2 (105 m.). The beds are best exposed at Halkin and Henblas, near 

 Holywell, and also at Trelogan and Gronant, near Prestatyn. They occur at the 

 same geological horizon as the Sponge-beds of the Yoredale series in Yorkshire, 

 that is, between the Carboniferous Limestone proper and the true Millstone-grit ; 

 but on the alleged grounds of a gradual lithological passage between the chert- 

 beds and veritable Millstone-grit, 3 Mr. J. A. Strahan, F.G.S., who has lately 

 surveyed the district, has included them as part of this latter division ; whilst Mr. G. 

 H. Morton, F.G.S., 4 regards them as Carboniferous Sandstone. Neither of these 

 geologists recognised the organic nature of the rock. Mr. Strahan has described 

 the chert as " probably a siliceous sediment of extreme fineness," and Mr. Morton 

 regards the strata as originally of sandstone, which has been, for the most part, 

 converted into chert. 



Microscopic sections of the chert collected from the different outcrops show the 

 presence of the same spicules as in the Yorkshire beds, and that this remarkable 

 series of chert rocks are built up of the integrated skeletons of siliceous Sponges. 

 As a rule, the spicules in these Flintshire cherts are not so favorably preserved 



1 ' Geology of Yorkshire,' part ii, p. 66. 



2 ' Mem. of the Geol. Survey, Explanation of Quarter-Sheet 79 N.-W.,' p. 18. 



3 Op. cit., p. 18. 



* * Proc. Liverpool Geol. Soc.,' vol. iv, pt. v, 1882-3, p. 393. 



