OF PEBBLES IN CONGLOMERATES. 487 



mass of pebbles and finely comminuted mineral matter, wbich has been consolidated and 

 folded into a series of closely compressed waves, the summits of which being denuded have 

 left the upturned nearly vertical beds as we now see them. We believe that such compres- 

 sion continued long enough after the beds assumed the vertical position to flatten and 

 indent the pebbles ; and that it not only produced the dynamical results, but, by the devel- 

 opment of heat within the mass of the rock, furnished the means for chemical changes. 

 We believe further that hard and rigid pebbles may, under such a force, be bent and flat- 

 tened, without becoming what would be called plastic, in the same way as fossils are often 

 distorted. 



Notes Relating to the Plates. 



The figures in Plates XVII and XVIII are of the natural size ; those on Plate XIX are of one half of the 

 natural size. The dotted line in each figure represents the direction of the strike ; the pressure is supposed to 

 have acted at right-angles to that line. PI. XIX, fig. 2, being sketched from a displaced fragment, has not the line 

 of strike drawn upon it. 



In PI. XVIII, fig. 1, the pebbles marked a and b, are coarse-grained, hard, and granitic ; the other stones are of 

 a softer material. 



In PI. XVIII, fig. 2, we have an example of pebbles considerably distorted, while immediately adjacent are schis- 

 tose fragments with a lamination at right-angles to the positioa it would have if produced by the force which has 

 bent the other stones. 



PI. XIX, fig. 1, shows a large, light-colored, fine grained, siliceous pebble, apparently fractured in bending. The 

 stones marked a and b, are of granite. 



Plate XIX, fig. 2, shows a large, fine grained, dark-colored rock, apparently distorted by being pressed against 

 the rounded pebbles, which are of granite. Between the larger round pebble and the upper stone occurs a group of 

 five distinct fragments, all of which seem to be distorted. The dotted line in this figure shows the direction accord- 

 ing to which the smaller stones, and the particles of the cement, are arranged. 



Published July, 1868. 



