CONSISTENCY OF GEOLOGY WITH SACRED HISTORY. 406 



times, these fragments are united by crystalline matter of a different 

 nature, forming the paste or cement, which holds them together ; at 

 other times, the paste is composed of nearly or quite the same mate- 

 rials with the fragments, but in a state of much finer division, and at 

 other times there is little interposed cement. 



But many of the rocks of this class are most palpably fragmentary, 

 and the fragments are of all sizes, from those that are scarcely visible 

 to the naked eye, to those whose dimensions are measured by in- 

 ches and even by feet. 



Instances. 



The brecciated marble of the Potomac, employed in the public buil- 

 dings at Washington, is a remarkably firm rock, composed of angu- 

 lar and ovoidal pebbles, the latter of which have evidently, received 

 their shape from friction in water. The cement is a more minutely 

 divided substance of the same kind, but calcareous matter is not ex- 

 clusively the material either of the pebbles or of the cement. 



The fragmentary rocks of Rhode Island, extending by Providence 

 to Boston, and which are very conspicuous in Dorchester, Roxbury, 

 Brooklyn, and other neighboring towns, are fine examples of early 

 formations of this kind. They are very interesting five miles east of 

 Newport, at a place called Purgatory, where a large mass of the rock 

 separated by the natural seams which are found in it, running parallel 

 for a great distance, and cutting the pebbles in two, has fallen out, 

 having been undermined by the sea, whose waves, when impelled by 

 storms, break and roar frightfully in this deep chasm. 



The pebbles are here chiefly quartz — they are ovoidal in form and 

 of every size from that of a birds egg to that of a common keg, and 

 they lie generally with their transverse diameters parallel. 



The pebbles of the fragmentary rocks about Boston are very va- 

 rious in their composition, obviously however the ruins chiefly of 

 primitive rocks. The pebbles, which there lie in the roads and fields 

 have proceeded from the disintegration of this pudding stone, 

 j The great sandstone deposit of the valley of the Connecticut pre- 

 i sents every variety in the size and form of the parts that have been 

 I broken up from previous rocks, — transported — more or less rolled, 

 1 and cemented into rock again. 



In East Haven, near New Haven, the rocks often contain massy 

 pebbles of granite, gneiss — mica slate and clay slate, and of the indi- 

 I vidual minerals of which they are composed. Water worn-pebbles 

 are in some places as common in these rocks as on the sea shore i 



