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112 Mr. Dana on the Analogies between the 
These views are further sustained by the action of heat on the 
walls of dykes. In subaerial eruptions of recent volcanic regions, 
the effect is usually slight; sometimes none is apparent, and at 
the most only for a few feet. When streams of lava have over- 
flowed tufas, it has baked them only for one or two feet and per- 
haps altered the color to red, for one or two feet more; but be- 
yond this, it is seldom that any effect is perceived. Clay will in- 
sulate the fused rock as completely in nature as when moulded 
into the shape of a furnace. 'The same effect must take place 
under water, except such modifications as may arise from heating 
the ocean itself, and the waters transfused through the stratified 
deposits; but these belong to the theory which I shall endeavor 
to sustain. The pressure of an ocean upon the erupted lavas, 
will not vary the result. Heat cannot be conducted to any extent 
except by fusing its way along, and in order to bake the rocks for 
twenty yards from adyke by conduction alone, the first fifteen 
at least must be in a state of complete fusion. There are numer- 
ous examples of alterations in rocks to a greater distance than this; 
but how few of them give indications that the walls of the cn | 
have been in fusion even for one yard ? 
But if the surrounding and permeating waters are heated at the 
time of a submarine eruption the heat may then be conveyed to 
great distances, and rocks may be discolored, baked or recrystal- 
lized, according to the temperature or extent and depth of the 
eruption. 
That waters are heated by submarine eruptions, is a matter be | 
observation. Dead fishes thrown up on the shores after eruptions 
are proofs of it.. But it is needless to waste words upon this point, 
for we know that water and fire cannot come in contact without 
this effect. It is evident too that the amount of heat imparted to 
the waters, will depend on the extent of the eruption, on the time 
of its continuance, and more especially on the pressure of the 
ocean above. For the dense waters at great depths require a 
high temperature for ebullition. 
To produce boiling the superincumbent waters must be so rais- 
ed in temperature,. that the vapor formed below, may pass UP 
through it and escape ; or in other words, ebullition will not take 
Place till the heat be so raised, throughout the whole by commu- 
nication from below, that the surface shall stand at 212° Fahr. 
At ‘no very great depth, hence, the waters might be raised to the 
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