596 PRINCIPLES OF STRATIGRAPHY 



as a sheet flood, the pebbles are more often merely moved back- 

 ward and forward without much overturning, or, again, the 

 pebbles are scarcely moved, but polished and worn by the sand 

 carried back and forth across them. 



Pebbles of glacial stream deposits are always rounded, since 

 in such deposits only the more massive rocks escape destruction. 

 The pebbles and coarser rocks of the White Mountain streams are 

 all well rounded, this being especially well shown in the flood plain 

 of the Saco River at Intervale. Destruction of all the weaker 

 type of rocks by prolonged river transport is also imminent, and 

 this is especially the case on large river flood plains or deltas. 

 Prolonged exposure of granitic or other coarse-grained igneous 

 rocks results in their disintegration, and thus by a process of assort- 

 ing, the pebbles may be reduced to a few fundamental lithologic 

 types, such as quartz, porphyry, etc. There are many older quartz 

 conglomerates with well-rounded pebbles, but free from fossils, 

 which appear to represent extreme cases of concentration. The 

 Millstone grit, the Pottsville conglomerate, and the Shawangunk 

 and other conglomerates are almost wholly composed of quartz 

 pebbles. In many cases these quartz pebbles are derived from vein 

 quartz, but in other cases they are probably a highly indurated 

 quartzite in which the individual grains have become obliterated. 



It is difficult to understand how extensive conglomerates, some- 

 times many hundreds or even a thousand feet in thickness, and 

 composed almost entirely of quartz pebbles embedded in quartz 

 sand, can have originated. If the material of the pebbles is vein 

 quartz, an enormous destruction of rock is indicated, since veins 

 form only a small portion of the rock mass carrying them. If 

 the material of the pebbles is an older quartzite, the problem is 

 less difficult. Again, in some cases the pure quartz pebble conglom- 

 erates may be formed of the reworked gravels derived from the de- 

 struction of older conglomerates. Thus the pebbles of the "yellow 

 gravel" of the coastal plain may be the product of the destruction 

 of the formerly much more extensive Pottsville conglomerate. In 

 this connection it is interesting to record that some of these 

 pebbles are fossiliferous, carrying corals of Siluric age. This may 

 represent a silicified limestone, the silica thus being of secondary 

 origin. Whether any such origin may be postulated for a con- 

 siderable portion of the quartz pebbles is doubtful. 



From our present knowledge of the subject, it seems that 

 no constant differences can be ascertained between river and shore 

 pebbles, both may be round or flat, and both may be well worn or 

 subangular, and form part of a mass of a very uniform lithic char- 



