THE CARBONIFEROUS SYSTEM. Ill 



and Sigillaria) 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. 



