56 GEOLOGY OF THE NAREAGANSETT BASIN. 



We thus see that in the erosional history of a region wherein the rain- 

 fall varies from zero to the highest measure which we can expect^ say over 

 100 inches per annum, we may look for a series of effects which will mark 

 themselves in diverse classes of debris. So long as the decay keeps in 

 general ahead of the abrasional work, the waste which goes forth through 

 the streams is likely to be of a finely divided nature, giving rise to clayey 

 slates as the natural product. If a stream yields abundant materials which 

 would form arkose, it is because the erosion is gaining on the decay. If 

 the work produces pebbles, the indication is that the mechanical erosion is 

 so great that wearing by solution plays no important part in the process. 



In this connection it will be well to note yet further that the large 

 production of pebbles within a short time can scarcely be accounted for 

 except on the supposition that the abrasion has been brought about by the 

 action of glacial ice. As the importance of this proposition has not been 

 appreciated by those who have dealt with the problems afforded by con- 

 glomerate deposits, we must note that there are but three ways in which 

 waterworn pebbles can be made in sufficient quantities to afford materials for 

 ordinary conglomerates. The first and practically the only effective means 

 by which pebbles can be extensively made — i. e., in large amount per unit 

 of surface over a large area — is by glacial action. Where the precipitation 

 of a country goes off as an ice sheet, every poiiion of the rocks over which 

 it flows, if the material be sufficiently hard, becomes a part of a vast 

 bowlder factory — ^for such, in fact, is all the base on which the glacier rests. 

 As the average thickness of the till covering in the glaciated district of this 

 country is not less than 10 feet, and as the greater part of this till is bowl- 

 ders, it is clear that ice in motion is specially adapted to forming such partly 

 rounded bits of stone. The materials in our eskers show how successful 

 the subglacial torrents, with their currents impelled by hydrostatic pi^essure, 

 were in completing the rounding of pebbles which the ice began to shape. 



When the rainfall of a country goes to the sea in a fluid state, the tor- 

 rent section of its river system, provided the rate of decay is swift, is likely 

 to be the seat of the production of a considerable amount of pebbly mate- 

 rial Yet the share of the energy of the portion of the rain water which 

 is then effectively applied to pebble making is but a small fraction of what 

 is used when the same amount of precipitation goes to the sea in the form 

 of a glacier. Moreover, the pebbles which are thus formed in ordinary tor- 



