THE POTOMAC AND RAPPAHANNOC. -2S9 



This mode of stratification appears to me to be an incon- 

 testable proof, that the wind has been the agent of accumula- 

 tions of the sand of which the stone consists, as it is now of 

 the sand hills of our present coast. For if it could be suppos- 

 ed, that the agitation of the surf, or of the whirls which occur 

 in all water running through an interrupted course, could have 

 caused this appearance, it would have occurred, as in cases 

 where there is no doubt of aqueous deposition, that the stone 

 would have separated more easily at the lines of stratification 

 than elsewhere. But such is not the case in this stone, for it 

 is as solid at the lines separating the strata as elsewhere. 



As the difference of granulation is exceedingly various, often 

 within a very small space, so is also the cohesion of the stone 

 very uncertain. Often with the fairest prospect of a hard sound 

 mass of rock, of great depth and thickness, the quarrier sud- 

 denly strikes into a mere friable sand bank. Quarrying is 

 therefore a lottery, in which the blanks are often more nume- 

 rous than the prizes. 



The quality of the stone, as a building material, is also in 

 other respects various. Of the stone most even in its grain and 

 texture, most pleasant to work, and of the most durable ap- 

 pearance, a great part cracks and falls to pieces, on exposure 

 to the sun and air, especially if rapidly dried, after being taken 

 from the quarry. Sometimes contrary to all expectation, the 

 frost tears it to pieces. — All of it expands when wet, and con- 

 tracts in drying. This property it seems never to lose. When 

 buried in the walls of a heavy building, it is controulcd by the 

 incumbent weight, but those blocks that are more at liberty, 

 either at one or both ends, are subject to this variation of size; 

 and the joints of the work open and shut, according to the 

 dryness or humidity of the weather. Window and door sellcs 

 therefore, which are confined at both ends, and free in the 

 middle, generally break, and the fissure opens and shuts alter- 

 nately, to the amount, when open, of one tenth of an inch 

 in a block of six feet. 



Below the freestone is found, on Potomac, most frequently 

 loose sand, sometimes a stratum of round gravel or pebbles, — 

 seldom clay, — very often loose stone very full of carbonated 



