OONDITIOIJ^^S OF PEBBLE MAKUvTG. 57 



rents are made in small quantities. No sooner does a torrent bed become 

 loaded with, this detritus than it ceases to be an effective factory of the 

 rounded bits of stone. Furthermore^ tlie pebbles whicli are formed in tor- 

 rent beds rarely attain the sea or any position where they can be built into 

 extended beds of conglomerate. Although I have inspected several thou- 

 sand miles of seashore, much of it along mountainous coasts, I have never 

 found a place where pebbles, such as are found in the conglomerates of 

 the Narragansett Basin, from a stream of any size were entering the sea. 

 It may, indeed, be regarded as rare for a stream to discharge into the ocean 

 pebbles exceeding an inch in diameter. To do such work it would have to 

 flow at a torrential rate at its very mouth, a condition which can be found 

 in certain fresh-water lakes, but is rarely seen on the ocean coast line. 



The third means of pebble making may be seen along the seashore 

 where the waves are attacking hard rock cliffs. In such conditions pebbles 

 are formed, but they are rarely accumulated in large quantities ; in general 

 the fate of coast-made pebbles is to be worn out by the action of the forces 

 which have shaped them. There are no agents whereby such marine 

 pebbles in considerable quantities can be carried out for any distance from 

 the shore. In rare cases ice forming along the coast is likely to inclose 

 some portion of the shore d(5bris; this shore ice may then drift out to sea 

 and there deposit the load of pebbles which it has rafted. This action, 

 though sufficient to strew the sea floor with shore-made stones, can not be 

 looked to as a means of accumulating conglomerates. 



The above-mentioned considerations make it clear that it is not easy 

 to account for the existence of widespread deposits of pebbles of consid- 

 erable size accumulated in massive strata, which in the case of the beds of 

 the Narragansett field contain perhaps more pebbles than exist on the beaches 

 of the Atlantic coast or in all the torrent beds of the Appalachian Moun- 

 tains. The easiest way, if not indeed the only way, to explain the forma- 

 tion of extensive conglomerates is as follows: On the surface of a land area 

 there must first be accumulated a considerable deposit of rock fragments, 

 such as is normally gathered at the close of a glacial period, or such as 

 occupies a region of high relief, scanty rainfall, and much frost work, after 

 the manner of large areas in the Cordilleras and in other parts of the world 

 where these conditions exist. If, now, such a fragment-strewn district is 

 graduall)^ lowered through the mill of a shore line either of the sea or of a 



