78 Professor Epwarp Hutt—On the Nature and Origin of Beds of Chert. 
of hornstein by F. Zirkel, seems to be applicable to some form of quartzite rather 
than to the gelatinous form of silica, of which the chert here treated of is composed. 
He speaks of it as “‘ Kin durchaus krystallinisches aggregat von eckigen und 
rundlischen Quartzkérnchen, von denen jedes wegen seiner abweichenden 
optischen orientirung im polarisirten Licht eine von der des Nachbarn verschiedene 
Farbe trigt.”* The definition of “hornstein” given by A. von Lasaulx, is 
somewhat similar.t Of the Memoirs of British authors who have treated of the 
subject of the silicification of organic forms,t perhaps the elaborate and exhaustive 
description of the silicified fossil corals in the West Indies, by Dr. Duncan, 
F.R.S., throws more light on the question treated of in this paper, than that 
of any other author. ‘The numerous and beautiful examples afforded by the corals 
of Miocene age of silicification in various stages, and in immediate proximity 
to others undergoing similar changes, throw much light on the causes and 
conditions under which transmutation has taken place in post-geological times. 
Dr. Duncan traces the completeness of the process in the case of many of the West 
Indian Corals, to the favourable conditions afforded by the intensity of heat and 
hight for chemical reactions. Repudiating the view that the silicification of the corals 
has been due to volcanic outburst, (as some have supposed) he observes—“ These facts 
rather tend to prove that the silicification of corals has been a slow process, which 
has had no other origin than in those chemical operations which are still in action, 
and that their greater or less intensity in certain favourable localities has produced 
siliceous fossils amongst those affected by the calcareous form of mineralization. 
Wherever a highly aerated sea containing silica in solution, acts on calcareous fossils 
at considerable depth and therefore under considerable pressure, there would appear 
to result a chemical transposition, and the presence of crystals of quartz, of 
homogeneous flint, and of hydrates of silica, is due to chemical influences which 
bear a relation to the geological changes in and about the reefs. In some cases 
not only the stony portions but the interspaces of the corals have been preserved 
in.silica.”§ . 
Mineral conditions of the Chert.—Observations under the microscope tend to the 
conclusion that the chert occurs in a gelatinous or colloid, rather than in a 
crystalline, condition. This view is clearly sustained when a thin slice is examined 
under the polariscope. The vivid play of colours exhibited by quartz in a crystalline 
condition (as that of granite, porphyry, or rock-crystal) is well known. But in the 
case of the chert specimens examined, the effect was very different, the result of 
* Die Mikroskop Beschaffenheit d.mineralien u. Gesteine, p. 108 (1873). 
+ Hlem. de Petrographie, p. 156, 1875. 
+ Amongst them should be specially named Dr. Bowerbank, F.R.s., whose researches on the origin of 
flints and silicified zoophytes and sponges, are to be found in the Proc. Geol. Soc., Vol. III., 278 and 
431; Trans. Geol. Soc., Vol. IT., 181 ; and Brit. Assoc. Rep., 1856, part 2, p. 63, &c. ; also Dr. Mantell, 
Wonders of Geology, 7 Edit., 18578. 
§ “ On the Fossil Corals of the West Indian Islands,” Part III. Quart. Journ. Geol. Soc. Vol. XX. p. 373 
