GENERAL DISCUSSION 269 



which is quite noticeable in freshly quarried rock, because the limestone 

 or shales that cover them are not discolored and yet penetrate holes and 

 cavities of the corroded surfaces. The particular reasons for interpreting 

 these as marine corrosion surfaces are because the caverns and burrows 

 extending down into the blackened limestone are filled with fresh marine 

 sediment and not with terrestrial residuum. The black coating of iron 

 and manganese is unlike that of weathered surfaces excepting perhaps 

 those of the deserts. The blackened surfaces are found to be encrusted 

 occasionally by marine fossil animals, and some of the encrusting fos- 

 sils are in turn corroded. The manner in which the black surfaces origi- 

 nated is believed to be simply this: that the deposition of terrigenous 

 material, chiefly clay, ceased for a time, and the lime deposit was mean- 

 while dissolved away by the sea-water as fast or faster than it was con- 

 tributed by sea-weeds, shells, and the like. The black surface deposit of 

 iron is a sort of residuum. 



CORROSION CONGLOMERATE 



In the conglomerates the pebbles have the same general surface char- 

 acteristics as those of the described corrosion surfaces. In the lowest 

 corrosion zone (number 1, figure 1) these are in fact small, blackened, 

 irregularly shaped pebbles associated with the corrosion surface, so that 

 their origin as loosened, corroded fragments is not far to seek. The other 

 conglomerates in zones numbers 4, 5, 6 and 8 (figure 1) are not directly 

 associated with the blackened surface of a corroded limestone stratum. 

 Their pebbles are in fact isolated in matrix of clay-shale or of shaly lime- 

 stone. The pebbles themselves are, however, limestone, with blackened 

 surfaces, the black stain penetrating with diminished intensity into them, 

 as in the case of the corrosion surfaces of limestone strata described. In 

 the corrosion zones numbers 5, 6, and 8 (figure 1) the pebbles are large. 

 They are mostly a few inches wide, but range in size from that of coarse 

 sand to pieces a foot wide. They are in fact too coarse to be explained 

 as incidental fragments from a corroded limestone surface, even if they 

 were directly associated with such a surface. Further explanation is 

 needed, therefore, to account for them. It will be sufficient to explain 

 one of these conglomerate beds, since the others are essentially like it. 



Conglomerate in the Galena formation 



general description 



Taking the conglomerate of the base of the Galena — zone number 8, 

 figure 1, as the best example — ^the matrix of this bed (number 6) is 

 XIX — Bull, Gbol. Soc. Am., Vol. 25, 1913 



