1875.] 99 [Roger?. 



action of a crushing force. Nor can it be doubted that chemical 

 changes have been wrought in the material in which the pebbles are 

 embedded, and even occasionally in the surfaces of the pebbles them- 

 selves, giving rise to the crystalline grains of magnetite scattered 

 through the former, to the mica-like scales which are found adhering 

 to the pebbles, as well as the cavities left by their removal, and to 

 the slight pitting or striation with which the pebbles are sometimes 

 marked. 



In regard to the generally elongated form of the pebbles in these 

 rocks, my observation of the breakers at various points on our coast 

 has led me to the conclusion that there is a marked difference in 

 the action of the impinging waves, due to differences in the slope 

 and smoothness, and the greater or less irregularity and contraction 

 laterally of the shores, so that while in some cases the movement is 

 chiefly a vertical whirling in the direction of the advancing wave, 

 in others it includes also various gyrations transverse to this. In 

 the former of these conditions the movement imparted to the peb- 

 bles at the shore would, it might be expected, grind them by mutual 

 attrition, and the wearing action of the sand, into oblong forms, 

 while in the latter conditions it would tend to bring them into 

 lenticular, or into more or less spherical shapes. The former of 

 these modes of action seems to prevail at many localities along the 

 Newport shores, and the latter is well exemplified by the lenticular 

 forms so abundant in the pebbles brought from the coast of New- 

 foundland. 



The flattened shape of many of the large masses may, to some 

 extent, be ascribed to the attrition operating upon them while partly 

 embedded and at rest, but chiefly to the laminated structure of many 

 of the fragments, causing them to break by concussion into flat masses 

 and to yield to erosive forces more rapidly in the planes of the lam- 

 inaa than in transverse directions. 



There is often a difficulty in determining the dip of these con- 

 glomerate beds, from the fact that in some of them the pebbles, in- 

 stead of lying with their longer sections parallel to the planes of 

 bedding, are placed partly edgewise to these planes, but by tracing 

 the separating beds or layers of sandstone it is usually possible to 

 discern the inclination of the strata. This oblique arrangement of the 

 pebbles resembles what is to be seen in similar accumulations of large 

 pebbles along the upper part of steep sea beaches of the present 

 day, or it may possibly have been caused, as has been asserted in 



