76 Professor Epwarp Hutt—On the Nature and Origin of Beds of Chert. 
of the “Upper Limestone” is highly coralline, and these organic forms together 
with the ossicles of crinoids are frequently preserved in solid chert. Amongst the 
corals, species of Lithostrotion are the most abundant. 
Mode of Formation of Chert—Views of previous Observers.—Having thus 
endeavoured to show the geological position of the principal cherty zone in the 
Carboniferous Limestone of Ireland, we proceed to the consideration of the 
questions :—(1.) How was this material formed? And (2.) when wasit formed? It 
was with the object of obtaining some satisfactory answer to these questions that L 
undertook the microscopical examination of specimens from various localities, 
receiving the ready aid of my colleague Mr. Hardman, rF.c.s. in the chemical 
branch of the inquiry. Before entering on the questions above raised, I shall 
briefly notice the opinions of previous authors. 
In attempting to enter upon a general historical review of the labours of other 
observers in this field, it will soon be admitted that the question of the origin 
and structure of Carboniferous chert, has not received sufficient attention from 
British, or other petrologists. Ina valuable contribution to the literature of this 
subject by Professor T. Rupert Jones, r.z.s., he expresses the opinion that one form of 
“chert” is a pseudomorphous replacement by silica of detrital carbonate of lime, 
amongst which very many small organisms, pieces of shells, tests, and encrinital 
ossicles, either remain visible, or have left cavities which have been washed out; in 
either case rendering the rock more opaque and coarse-grained than ordinary flint and 
hornstone.. Dr. W. K. Sullivan, observes,t that “in most limestones concretionary 
masses of silica occur. In the older rocks these concretions are termed hornstone 
or chert. In the chalk they are flints which according to Ehrenberg consist for the 
most part of the remains of infusoria. It is probable that some of the hornstones 
of the Carboniferous Limestone are also of organic origin, but that all are not so is 
proved by the partial conversion of fossil corals into hornstone, a fact which shows 
that the hornstone is due to a subsequent pseudomorphosis.” 
The views expressed by the two authorities above quoted are we believe fully 
borne out by microscopical and chemical examination, and represent very fairly the 
most recent opinions of British Geologists. 
The frequent occurrence of beds of chert in the limestone formations of North 
America, haS naturally attracted the notice of the naturalists of that Continent. 
Chert abounds in the limestones of the “ Bird’s Eye” and Black River divisions of 
the Trenton Group, (Lower Silurian), where it occurs in interrupted beds and 
masses of two or three inches in thickness. It also characterizes in an especial 
manner the limestones of the “Corniferous formation,’+ (Lower Devonian), and 
occurs in the limestones of the Carboniferous system of Tennessee, Illinois, and 
* “« On Quartz, Chalcedony,” &c. Proc. Geological Association, Vol. IV. No. 7. 
* Jukes’ Manual of Geology, 3 Edit. p. 60, 1872. 
{ Logan, Geology of Canada, p. 628. 
