THE CARBONIFEROUS SYSTEM. 111 
and Stgillaria) rarely of any great length, but frequently showing their 
markings with distinctness; the Trigonocarpa exhibiting only the nut 
with its nucleus, the external fleshy envelope and the delicate wings of 
the nut having been all removed by attrition. Occasionally the sand- 
stones and pudding-stones of the Conglomerate are interstratified with 
layers of argillaceous shale, especially at the top of the formation, and 
here we sometimes find some fern fronds. Such exceptional cases as 
these are plainly the products of local causes, which, in the emergence 
of the continent and the supervention of the terrestrial on marine con- 
ditions—in other words, the succession of the Coal Measure epoch to the 
Conglomerate epoch—occasioned the Coal Measure conditions to be 
locally reached before they generally prevailed. 
In western Pennsylvania—Warren, Kinzua, etc.—the Conglomerate 
contains great numbers of fossil mollusks near its line of junction with 
the Waverly, and I have noticed the same thing in a few localities in 
Ohio. These fossils include several species, all of which, so far as I 
know, are found in the underlying strata, and they simply indicate that 
in certain localities the change of physical condition recorded in the 
different lithological characters of the two deposits took place more 
gradually than elsewhere. 
Some years since, at a meeting of the American Association, the geolo- 
gists present were much puzzled by some specimens of the Conglomerate 
exhibited by Prof. Brainerd, of Cleveland, in which the impressions of 
the stems of plants were as distinctly transmitted to the quartz pebbles 
as to the interspaces of sand. Prof. Brainerd argued from these speci- 
mens that the pebbles were of concretionary origin, and that they bore 
the markings of the bark of plants because they had been formed in 
contact with such bark. The recent experiments of Thenard, which 
show that humic acid renders silica readily soluble, afford an easy solu- 
tion of the problem, and confirm the view taken by the writer upon the 
occasion referred to above, viz., that the pebbles had been dissolved away 
where in contact with the plant. The proof that the pebbles of the 
Conglomerate are not concretionary is abundant and conclusive. In 
some localities many of them are composed of something else than 
quartz; silicious slate showing stratification being a common material. 
Conglomerate pebbles composed of chert containing fossils I have already 
referred to. 
The economic value of the Carboniferous conglomerate is very great. 
Throughout the whole area occupied by its outcrop it furnishes a more 
or less desirable building stone, and almost exclusively supplies the 
want of such material to many of the communities resident on this area. 
