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fluid, which has perfectly cemented together the several pebbles, 

 which had before been connected, by interposed particles of sand, 

 clay, &c. 



Frequently the external surface, or coat, of the pebble, will be 

 formed by a hard silicious crust ; a small part of which being re- 

 moved, shows that the remaining substance of the pebble is a soft, 

 pulverulent, sometimes argillaceous, and sometimes calcareous, 

 matter. In this instance it appears that the silicious matter has 

 introduced itself only to a certain depth in the substance of the 

 pebble ; the rapidity of concretion, or crystallization, having pre- 

 vented any subsequent introduction of the fluid. A pebble, now 

 before me, seems plainly to show, that the process, by which the 

 induration of these pebbles is accomplished, is not suddenly per- 

 formed. A part of this pebble is covered with a crust similar to 

 that with which these pebbles are commonly invested, which termi- 

 nates by a strong defined ridge, within which is a substance, the 

 appearance of which marks it to have been the soft internal 

 substance, which had been deprived of its crust, previous to the 

 thorough penetration of the silicious matter. In this are stuck 

 about twelve small black silicious fragments, which, by a subse- 

 quent infiltration of silicious liquor, are now firmly connected in 

 one solid mass, with the original soft internal substance of the 

 pebble, a depression existing in one part, formed by one of these 

 fragments, which has since separated. 



Organic remains, which originally contained a considerable pro- 

 portion of calcareous earth, are frequently found imbedded, or en- 

 veloped, in these pebbles ; and are often so thoroughly pervaded, 

 in some parts, by the silicious impregnation, as to be absolutely 

 silicious ; but sometimes, the calcareous earth is, in other parts of 

 the same stone, still so predominant, and is united with so much 

 carbonic acid, as to effervesce strongly on the application of even 

 vinegar. Strong evidence of the slow accretion, and of the long- 



