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durability, presents at the distance of several miles to the south a 

 crumbling mass, whose value consists in its being useful as a sub- 

 stitute for sand, or furnishing a good material for the manufacture 

 of fire bricks. It may, therefore, be found a guide of some value in 

 the selection of rock for flags or building purposes, to choose such 

 as, lying in the vicinity of a heavy vein of quartz, and intersected by 

 smaller ones throughout its substance, is likely to possess the com- 

 bined advantages of great hardness and durability, and a compara- 

 tive facility in being quarried, in virtue of the cross joints by which 

 it most generally and spontaneously divides itself. 



In addition to the rocks here described as occurring particularly 

 in the South-west mountain and its vicinity, various others are pre- 

 sented both to the south and north of the localities to which the 

 profile may be considered as referring. Thus, in Orange and in 

 Nelson and Amherst counties, as well as in the neighbourhood of 

 Scottsville, in Albemarle, and interruptedly in many other places in 

 the same general range, a very interesting rock is seen, consisting 

 of fragments sometimes angular, sometimes more or less water 

 worn, cemented together by particles of sand, and occasionally a 

 small admixture of carbonate of lime. This singular conglomerate 

 has evidently been in part derived from the greenish- blue rock pre- 

 viously described, with which its larger pebbles or fragments are 

 obviously identical — and in part from the sandstones and occasion- 

 ally the limestones of this region. It is in fact the representative, 

 in this portion of the state, of the Potomac marble, and some of it 

 when polished would present a surface of equal variety and beauty. 

 The occurrence of this rock, as here described, obviously marks an 

 epoch of violent action, in which the neighbouring strata, of which it 

 may be considered as embodying the ruins, were broken into frag- 

 ments, and these subjected for some time to the rounding agency of 

 water, at the bottom of which the coarser and the finer sediments 

 were at length consolidated into rock. A curious fact, for the first 

 time observed by my brother, Professor H. D. Rogers, would seem 

 to show, that in the composition of the Potomac marble, fragments 

 of limestone may be seen, referable to no nearer source than the 

 great valley west of the Blue Ridge. In some of the columns in the 

 senate chamber at Washington, which by their beautiful polish 

 enable the observer as it were to look into many of the rocky frag- 

 ments of which they are composed, ho detected distinct impressions 



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