Mr. Matter on the Dynamics of Earthquakes. 85 
Bouguer, Don Ulloa, and other contemporary or slightly anterior authors. His 
hypothesis is, however, purely gratuitous; we have not a particle of evidence 
that any large tract of the earth’s surface ever is afloat upon, much less buoyed 
up by, and elevated upon, vast masses of clastic vapour or gases. The only evi- 
dences we have of subterraneous vapour playing any part at all in the forces of 
elevation, are at the very foci of volcanic action, by the projection of solid masses 
from craters, the occasional splitting and blowing out of the sides of these, the 
spouting of geysers, &c.; but these, however grand when considered alone— 
‘‘magna ista quia parvi sumus”*—are only minor phenomena in the great machi- 
nery of elevation of the earth’s crust; and if any considerable tracts of surface, 
even in the neighbourhood of volcanoes, were afloat upon elastic vapour, rapid 
and perceptible falls of surface, ending with shocks, long continued and irrepres- 
sible blasts of the liberated gases, and other evidences of sudden depression, would 
be felt at those moments of the eruption when, by one of those mighty explosions, 
the caverns below were eased of their pent-up winds, and their roofs again dropped 
suddenly into contact with their molten floors. But no such facts have ever been 
observed ; in all eruptions, however violent, the principal phenomena indicate the 
steady, upward pressure of liquid, but not wriform, matter from below: the pres- 
sure and all its effects are hydrostatic ; and explosions, or sudden evolutions of elastic 
vapour, appear merely as the by-play, produced by the casual coming in contact 
of the heated materials with water, or with solids evolving vapours or gases by 
decomposition. 
When the great rents at last open, liquid lava flows out, either quietly or 
attended with eructations of elastic matter ; but these are disproportionately small 
in relation to the volume of liquid. Even when vast “crevasses” have opened up 
the bowels of volcanic mountains to their very bases, no great evolvitions of elastic 
matter have ever been recorded. Thus, in the eruption of Etna, in 1669, a 
fissure or “ crevasse” of about six feet in width, and of unknown depth, suddenly 
opened in the plain of St. Lio, at the very base of the mountain, and ran up to 
within a mile of its summit—a fissure of twelve miles in length ; it was soon 
succeeded by five others, nearly as large, and nearly parallel to it. Amidst the 
crash of riven rock, and rolling thunders from the hell below, heard forty miles 
* Seneca, Ques. Nat. 
