58 GEOLOGY OF THE NARRAGANSETT BASIN. 



considerable lake, the chance for the formation of normal conglomerates 

 will be provided. The unorganized debris of the surface will be taken to 

 pieces and recomposed into stratified beds, as is now being done with the 

 glacial debris along the shores about the North Atlantic. In this formative 

 process the pebbles are likely to be changed in shape and assorted as to 

 size in a way which at once distinguishes the strata into which they are 

 built from the beds of till or of residual breccia from- which the fragments 

 were derived. 



There are certain tests of some value in distinguishing the conglom- 

 erates made from rearranged glacial materials from those which owe their 

 formation altogether to marine action. Pebbles made from fragments 

 which have long been separated from the bed rock are generally, unless 

 they be of quartz, much affected by decay; they contrast distinctly with 

 the fresh quality of the ordinary glaciated pebbles. As I have observed 

 in the southern part of this country, as well as in southern Europe, the 

 detrital waste which comes into the streams is generally so penetrated by 

 decay that it can not be made into pebbles; if perchance it holds together 

 in the shaping, the eye at once separates the bit from those which are made 

 from freshly riven stone. 



Where pebbles are made by wave action alone, it is a notable fact that 

 they exhibit very little diversity in form; they are almost invariably sphe- 

 roidal, and when they are accumulated in considerable numbers the litho- 

 logical diversity of the material is small. Although glaciated pebbles are 

 apt to be somewhat altered from their original subangular shapes as they 

 pass through a surf line, many of them, as we may note along the New 

 England coasts, will withstand a deal of hammering without losing the dis- 

 tinct mark which the ice work impressed upon them. Moreover, taken col- 

 lectively, whether in the original till or in the partly masked shore deposits, 

 they commonly exhibit a large petrographical range of material. The facts 

 which are available for the interpretation of conglomerates show that those 

 of the Narragansett Basin are of what we may term secondary glacial 

 origin. This is indicated by their frequent — indeed, we may say usual — sub- 

 angular form, their petrographical variety, and the very small amount of 

 decay which had affected the rock masses after they left their original bed- 

 ding places and before they were deposited in the situations in which we 

 now find them. 



