THE POTOMAC AND RAPPAHANNOC. '281 



and diminish the value of these quarries exceedingly, They 

 are found from the size of a pea, to many inches in diameter. 



Pebbles, large and small, of quartz, sand stone, granite, whin, 

 rounded by attrition, and amorphous lumps of quartz. 



Pyrites, or lumps of marsh mud mixed with sulphat or 

 sulphuret of iron, efflorescing in the air. Often when one of 

 these pyrites happens to be concealed near the surface of a 

 wrought stone, so that the air and water may reach it, it swells 

 and bursts the stone, thereby defacing the work. This is ano- 

 ther disadvantage in using it. 



Nodules of von ore in sand, these nodules dissolve in the air 

 and water, and stain the stone disagreeably. In a spherical 

 hole of the stone, I once found a nest of very beautiful paral- 

 lelopipedal crystals, quite transparent. I had no opportunity 

 of examining them chemically. 



Wood, from trunks and branches of trees of large size, to 

 small twigs, either entirely carbonated, or the wood carbonat- 

 ed and the bark in a librous state, so as to have the appearance 

 of a net, and a considerable degree of tenacity ; or the bark 

 fibrous, and the wood in a state quite friable; or the wood re- 

 placed by pyrites, which effloresce in the air* ; or in cavities, the 

 sides of which have the impression of branches, in minute rami- 

 fication, and are lined with a pellucid crust, probably calcareous 

 spar. This latter evidence of the admixture of wood, is to be 

 found chiefly near Fredericksburg. 



Iron, appearing in stains, either of masses, or in dark fer- 

 ruginous spots and clouds; and clay, infused through the whole 

 mass, with probably an infusion of lime, which appears to be 

 the cement by which the sand particles are held together; for 

 the acids indicate no existence of carbonate of lime, and I have 

 not yet been able to submit the stone to any chemical exami- 

 nation. 



* I had a piece of this species of pyrites, in appearance the branch of a tree, with a 

 small twig attached to it ; about 3 inches long, and % of an inch diameter, very hard and 

 heavy, I carried it in my pocket for a fortnight, and then threw ii into a small boxinrm 

 office, containing drawing instruments. It remained there for two years, but last summer 

 during very damp and hot weather, it effloresced, and fell into powder, corroding and if 

 juiingmy instruments exceedingly. 



