CARBONIFEROUS CONGLOMERATES. ,35 



conglomerates, their wide distribution, as well as the general absence of 

 fossil remains, are best explained on the supposition that the erosion 

 took place in an ice time, being effected by the glaciers or by the currents 

 of living water which coursed beneath them. In no way save by com- 

 paring this ancient work with that now in progress in glaciated regions can 

 we well account for the deposition of the Millstone grit of the Southern 

 Appalachians. 



As we go northward from the valley of the Tennessee the Carbonifer- 

 ous strata show an increasing amount of pebbly material which has been 

 derived from the undecayed bed rocks. As elsewhere noted, these rocks 

 indicate that there was at the beginning of the period a considerable thick- 

 ness of decayed material, but before long the erosive agents had removed 

 this friable mass, and thereafter the supply of pebbly matter, vast in amount, 

 was obtained by the breaking up of bed rocks which show no evidence 

 that they had been affected by superficial decay. As we go yet farther 

 north, in the next field where the rocks of this age appear, i. e., in the region 

 about the south shore of the St. Lawrence, the thickness of the conglomer- 

 ate even exceeds that of the sections hereafter to be described in the north- 

 em part of the Narragansett Basin. In a word, the facts make it evident 

 that the Carboniferous period of the eastern part of North America, like 

 certain other periods, was one of exceedingly rapid alternations between the 

 conditions which favored the development of marsh vegetation and others 

 under which the accumulations of coarse sedimentary deposits went on with 

 great rapidity. 



Although there are instances in which a torrent may accumulate a 

 large detrital cone composed of bowlders and pebbles, I know of no geo- 

 logical machinery now at work on the earth's surface, or which can reason- 

 ably be supposed to have operated in the past, except glaciation, that is 

 competent to produce such immense masses of coarse detritus as are contained 

 in these conglomerates, or bring them into position where water action can 

 effect their arrangement into beds. The area of the deposits lying on the 

 two sides of the old Appalachian axis probably now exceeds 60,000 square 

 miles; the average thickness of the section is certainly not less than 2,500 

 feet; so that the amount of matter of a prevailingly coarse nature which 

 was laid down along the old Appalachian ridge in a period apparently of 

 no great duration was not less than 20,000 cubic miles, and probably was 



mon xxxiii 5 



