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



show, however, that in all eases the silica present is in the insoluble 

 form, and he seems to have been puzzled to reconcile this fact 

 with the preconceived notion that it was in the amorphous or 

 soluble condition. The sections of the Irish Chert which I have 

 examined between crossed Nicols all show the optical characters 

 of chalcedony and quartz, and thus substantiate the accuracy of Mr. 

 Hardman's analyses ; it is therefore probable that Prof. Hull's 

 statement that the silica is now in a colloid condition is founded 

 on some mistake. 



In order fairly to ascertain the microscopic structure of the Chert 

 I had sections prepared, from specimens obtained from every locality 

 I visited, including Dublin. The localities comprised Dunamase, 

 near Maryborough, in Queen's Co. ; Bonnet's Eath, near Kilkenny ; 

 Keishcorran, near Ballymote ; Knock-na-Eea, and Ben Bulben in 

 the County of Sligo, and also Benachlan, Florence Court, near 

 Enniskillen. 



Nearly all the sections were prepared from the prevalent dark 

 Chert. This, when viewed by reflected light under the microscope, 

 showed a diffused, cloudy, bluish-white reticulation on a dark 

 ground. By transmitted light the bluish portion becomes trans- 

 lucent, and when the section is very thin, transparent. It is 

 resolved into microscopic sponge-spicules, confusedly intermingled 

 together, whose individual outlines can be traced with varying 

 degrees of clearness. In transverse section the spicules appear as 

 well-defined circles, with, in many cases, a central spot indicating 

 the axial canal of the spicule. In longitudinal section, the larger 

 spicules exhibit two parallel lines with a clear space between them, 

 but viewed in this direction the individual forms are much less 

 distinct than when seen transversely. In most of the sections under 

 examination, sponge-spicules are the only organisms visible, in some 

 there is a slight admixture of specimens of the genus Fenestella and 

 other Polyzoa, the cells of which are now filled with either chalce- 

 dony or quartz, rarely also there are fragments of minute Brachiopod 

 or Entomostracan shells, and one or two microscopic ossicles of 

 Crinoids, but I have not recognized Foraminifera in my sections. 



The dark material in the Chert is for the most part disposed in 

 wavy bands, following the plane of bedding. Some of it consists of 

 very minute opaque crystals, but for the most part it has a cloudy 

 appearance. It appears to me to be partly of a carbonaceous and 

 partly of a ferruginous character. As already mentioned, this dark 

 material disappears in the weathered outer crust of the Chert, and 

 even the central portion of black Chert beds becomes bleached after 

 exposure. 



Sections of the light-brown Chert from Benachlan contain, in 

 addition to spicules, numerous well-defined microscopic cubic and 

 rhombic crystals, either entirely or partially transparent or opaque ; 

 they vary from -02 to '075 mm. in diameter ; similar crystals are 

 also present in the dark Chert, but in fewer numbers. In this brown 

 Chert the spicules have a brownish cloudy aspect in transmitted 

 light; their outlines are extremely jagged, showing that solvent 

 action has reduced them to mere skeletons of their original forms. 



