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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 
removed, 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 j 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 in- 
duration of these pebbles is accomplished, is not suddenly performed. 
A part of this pebble is covered with a crust similar to that with 
which these pebbles are commonly invested, which terminates 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 subsequent 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- 
