THE CONGLOMERATE ROCK OF THE COAL MEASURES.
On page 57, in describing this rock we were induced to follow the theories of Rogers and Lesley in regard to its origin and formation,—first because we have much confidence in Professor Lesley's opinions ; and, second, because the argu ments presented seemed to justify the conclusion. But even Lesley is undecided on this question. He says, in his Manual of Coal, page 73 : " But after disposing of all these arguments, concretionary quartz remains a possibility, and some as yet unknown method of explaining its susceptibility to impressions is a great desideratum." Professor Brainard, of Cleveland, maintains, by many strong arguments, that the pebbles of the conglomerate rock are concretions, and the precipitate of a solution of silex, or feldspar and mica, formed by the chemical action of lime, &c. These pebbles are always smooth, as if formed by detrition, like those of a shore or stream ; but, unlike the latter, they present no angular surfaces ; those which are not wholly smooth are crystalline; they are generally flattened, and always imbedded on their flat surfaces, as shown in figure 5. They are often pitted as if marked while in a soft condition, and sometimes contain impressions of plants, which must have been produced while the pebbles were in a soft condition. These and other considerations, which we will state below, induce us to conclude the conglomerate rock to be precipitated during a time of volcanic quiet, and that the white pebble are concretions of silex.
The immense horizon of this rock ; its uniform spread, yet gradual depreciation both in thickness and coarseness westward and northwestward, indicate its source as identical with that from which all or most of the material filling the Great Basin was derived. We said, on page 57, that it evidently could not have been of vol canic origin, since there appears to be little or none of the pure igneous rocks in the mass." This, we must state, is erroneous in fact, though correct when con sidered as a purely sedimentary formation, produced by the detrition of existing rocks ; but we have persistently advanced the theory of the volcanic origin of all our rocky strata, whether clay, slate or sandstone ; yet these rocks contain no evi dence of direct volcanic eruption. The Red-shale, on which, and in which the con glomerate pebbles are frequently imbedded, does not appear to be derived from volcanic sources, and yet that such is its source cannot be successfully denied, though at the time of its precipitation violent volcanic action did not take place except at intervals, but the ashes, dust and smoke of their smouldering quite filled the air and the waters with a fine sediment, intermixed with the coarser strata of intermittent eruptions. To be consistent, we must therefore repudiate the theory of detrition, as stated on page 57, and accept the more natural one of concretion, to account for the white pebbles in the conglomerate rock.