Dr. G. J. Hinde — Organic Origin of Chert. 439 



obscure forms of foraminifers ia section, coralline structures ? and 

 bivalves (brachiopods) with crystalline silica in interiors," etc. In 

 the original of the figure, however, sections of crinoid stems do not 

 appear, and it is evident that the forms described as "circular disks 

 of crinoids with dark central nuclei " are really transverse sections 

 of sponge-spicules, and the central nuclei are sections of their axial 

 canals ! These are abundant enough in the section. A similar 

 mistake vitiates the descriptions of the other sections, and the 

 supposed Crinoidal stems are, in most instances, the spicules of 

 sponges ! When a chance section of the ossicle of a Crinoid does 

 occur in Prof. Hull's slides, the distinction between it and a sponge- 

 spicule is readily apparent, for not only is the Crinoid fragment 

 much larger, but it also exhibits the minute cribriform structure 

 peculiar to Echinodermal organisms, which is so well known to 

 every tyro in the microscopical study of organic rocks, and serves 

 to mark them off from sponge-remains. Prof Hull has charged me, 

 and erroneously, with mistaking sections of Crinoids for sponge- 

 spicules, but it is evident that he was himself really committing 

 the converse mistake of describing the sponge-remains in his own 

 specimens as portions of Crinoids ! 



As Prof. Hull's specimens were selected without any reference 

 to their composition, and as they have been brought forward as 

 evidence of the inorganic origin of the silica of the Chert, the fact 

 that some are found filled with siliceous sjDicules may fairly be 

 taken as a strong argument from a perfectly independent source 

 that the Carboniferous Chert is really derived from the silica 

 of sponge remains. 



The Chert in the Irish Carboniferous Limestone is mainly 

 developed in the Upper Division or Upper Limestone, which ^ Prof. 

 Hull regards as the equivalent of the upper portion of the Carbon- 

 ifei'ous Limestone in England, the shale beds intervening betvireen 

 this Upper Limestone and the Millstone Grit being considered by 

 him as the representatives of the Yoredale beds of Yorkshire. As 

 a fact, however, the Yoredale series in its typical region mainly 

 consists of beds of limestone associated with beds of Chert, often of 

 considerable thickness ; these limestone and chert beds are of a 

 similar character to those of the so-called Upper Limestones of the 

 Irish series, and it is therefore reasonable to conclude that these 

 Chert-bearing Upper Limestones are the equivalents in the geological 

 scale of the Chert-bearing limestones in the Yoredale series of 

 Yorkshire and North Wales. 



According to the reports of the Irish Geological Survey, beds of 

 Chert are very generally distributed in the Upj^er Limestones of the 

 south-western districts of Tipperary, Limerick, Kerry, and Cork, 

 but I had no opportunity of visiting these counties, and my obser- 

 vations were limited to the Chert in the Limestones in Queen's 

 County and Kilkenny to the south, and in the Counties of 

 Fermanagh and Sligo to the north-west of Ireland, from whence 

 Prof. Hull obtained the specimens on which he has founded his 



^ loe. cit. p. 73. 



