Dr. G. J. Hinde — Organic Origin of Chert. 441 



SO to 50 feet (9 to 15m.) in thickness. As the total thickness of the 

 Upper Limestone series, in which the Chert is mainly developed, 

 is from 600 to 800 feet, there is probably an aggi-egate thickness of 

 100 to 150 feet of Chert (30-45m.), and in addition to this there 

 is a considerable thickness of Chert in the Middle or Calp series of 

 Limestone. 



There is considerable uniformity in the character of the Chert in 

 the Carboniferous series of Ireland. It is, for the most part, a 

 compact, dark rock, it breaks with a conchoidal or splintery fracture, 

 is too hard to be scratched with a knife, and, as a rule, gives no 

 action with acid. The fractured surfaces are usually dull, sometimes 

 with a faint lustre, resembling the so-called Lydian Stone. 

 Horizontal lines of banding are not often shown in it. The beds 

 are sometimes traversed by minute thread-like fissures, now infilled 

 with quartz. The upper and under surfaces of the beds weather into 

 a light grey or whitish siliceous crust, in some cases nearly an 

 inch (20mm.) in thickness, of a porous granular material, harsh to the 

 feel, and adhesive to the tongue, which strikingly contrasts with the 

 black compact Chert of the central portion of the beds. In cases 

 of extreme weathering, the entire mass of the Chert is converted 

 into this porous rock, it then has a pumice-like aspect, and is 

 relatively very light. 



In the case of cherty limestones, in which the shells of Brachio- 

 pods, Crinoid stems and other calcareous organisms have been 

 imbedded, the calcareous structures are entirely removed by weather- 

 ing, and the impressions of the fossils are distinctly shown in the 

 siliceous matrix. 



In some cases the Chert has a light brownish tint ; it is equally 

 as hard as the black Chert, and weathers out in a similar manner. 

 In Queen's County and Kilkenny some of the nodular masses, 

 and beds of Chert as well, are of a grey or bluish grey tint, the 

 beds are much jointed, and thin films of calcite have formed in the 

 joint-planes. Occasionally the siliceous rock occurs in the granular 

 porous condition in the limestones, it does not appear to have 

 reached the stage of compact Chert. When treated with acid or 

 when weathered out naturally, the siliceous portions stand out from 

 the limestone in the same way as the porous crust on the solid 

 Chert. 



I have not had any chemical analysis of the Chert prepared, those 

 carried out by the late Mr. Hardman^ being sufficient to show 

 its mineral composition. In several of these analyses the proportion 

 of insoluble silica varied from 90 to 95 per cent, the residue con- 

 sisting of carbonate of lime, iron, alumina aiad magnesia. In less 

 pure Chert the proportion of silica diminished, whilst that of 

 carbonate of lime increased, and some of the rocks analyzed were 

 rather limestones than Chert. Prof. Hull states from his micro- 

 scopic observations that the silica of Chert occurs in a gelatinous 

 or colloid condition.^ The chemical analyses of Mr. Hardman^ 



1 Scient. Trans. Eoyal Dublin Soc. vol. i. 1878, pp. 86-91. 



2 Op. cit. p. 84. 3 Op. cit. p. 92. 



