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continued application of the indurating matter, in a state of solu- 
tion, to these pebbles, during, and even subsequent, to their having 
assumed the general form which they now possess, may be found in 
a variety of the silicious pebble, which appears to have derived its 
shape from an organic body, apparently a species of coral, which 
it has invested. In this pebble, the holes are frequently left, 
through which some parts of the contained body have projected; 
but frequently it will also be found, that these holes are filled up, 
in various degrees, by silicious matter, showing the strongest marks 
of its deposition, from an aqueous solution, subsequent to the for- 
mation of the stone ; sometimes it surrounds and penetrates the ex- 
traneous body itself, and at other times fills up the hole, so exactly 
as to leave externally only a circular seam ; not much differing from 
the appearance of a cicatrix. Another proof of the subsequent 
addition of silicious matter, in solution, to pebbles, offers itself in 
the junction of the fractured parts of the boulders of sand-stones, 
for instance, by the interposition of silicious matter; thus, fre- 
quently, will the fragments be seen to be united by a very minute 
seam ; not such as might be supposed to have originally existed in 
the rock of which this stone is a boulder ; but a slight crack pass- 
ing through its substance, and which yields to different degrees of 
force in different pebbles, according to the various degrees of ag- 
glutination it has experienced. Having laid before you some of 
the circumstances which the pebbles, forming the vast beds of 
gravel so frequent in this island, offer in proof of the influence of 
water m the process of silicious impregnation, it becomes neces- 
sary, in the next place, to determine, whether agates, and agatine 
nodules, derive their formation from the influence of the same 
agent; since it has been eagerly contended, by Dr. Hutton and 
Mr. Playfair, that their formation depends on the injection, in a 
state of fusion by heat, and subsequent cooling, of silicious matter. 
This examination is rendered the more necessary, by the latter 
