116 3 Mr. Dana on the Analogies between the 
especially salt water, which contains so largely of alkaline salts, 
The siliceous waters of the Iceland Geysers are examples of such 
effects at the present time; and judging from the deposit around 
these boiling springs, silica is here in solution and not siliceous 
compounds. The dissolving of silica must therefore be one of 
the first effects of the heated waters, and this petrifying or solidi- 
fying earth, as well as heat, is distributed through the adjoining 
rocks.. In regions of eruptions, numerous quartz veins or silici- 
fied fossils often occur that may be imputed to this source, and 
the hardening of the sandstone or clay may depend to as great 
an extent upon the distributed silica as upon the heat. The silica 
is not all introduced from the external. waters ;—that in the clays 
themselves becomes partially dissolved and is redeposited as the 
water cools; in the same manner as the common waters of the 
ocean by washing through a bed of coral sand on the shores will 
after a while dissolve and deposit lime enough to cement it into 
a compact limestone. Why are the more ancient sandstones and 
grit rocks of our globe so much harder and so much more thickly 
intersected by quartz veins, if it be not due to. the heated sili- 
ceous waters to which they have been for so many ages at various 
eruptions exposed? and ata period—that of their formation— 
when probably volcanic eruptions were more violent and nume- 
rous than now ? As 
The heated waters at an eruption of greenstone, holding silica 
in solution which they have taken from the siliceous materials at 
hand, are in a favorable state also for the formation of the many 
zeolites and other trap minerals. In some amygdaloidal cavities 
the waters, as they cool, deposit silica alone. - From a dense gela- 
tinous solution layer is deposited on layer, and a coating of chal- 
cedony or agate formed ; afterwards the remaining silica, now in 
less dense solution, enters more slowly into regular crystals. The 
waters that penetrate to other cavities contain compounds of sil- 
ica. Percolating through the rock it takes up lime, soda, potash, 
alumina, iron, &e. or the elements of the constituent minerals, 
and the solution thus formed fills the open amygdaloidal cavities, 
which finally on evaporation yield the various crystallized mine- 
rals common in these cavities. I do not attribute these erystalli- 
zations in all instances to the same period in which the eruption 
of the containing amygdaloid took place, and they may have 
formed at a much later period. At some subsequent erup- 
