49G 
ELEMENTS OF GEOLOGY. 
ment of the included gases, and thus forms scorim^ other 
portions being reduced to an impalpable powder or dust. 
The showering down of the various ejected materials round 
the orifice of eruption gives rise to a conical mound, in which 
the successive envelopes of sand and scoriae form layers, dip¬ 
ping on all sides from a central axis. In the mean time a 
hollow, called a crater^ has been kept open in the middle of 
the mound by the continued passage upward of steam and oth¬ 
er gaseous fluids. The lava sometimes flows over the edge of 
the crater, and thus thickens and strengthens the sides of the 
cone; but sometimes it breaks down the cone on one side 
(see Fig. 585), and often it flows out from a fissure at the base 
of the hill, or at some distance from its base. 
Some geologists had erroneously supposed, from observa¬ 
tions made on recent cones of eruption, that lava which con¬ 
solidates on steep slopes is always of a scoriaceous or vesicu¬ 
lar structure, and never of that compact texture which we 
find in those rocks which are usually termed ‘‘trappean.” 
Misled by this theory, they have gone so far as to believe 
that if melted matter has originally descended a slope at an 
angle exceeding four or five degrees, it never, on cooling, ac¬ 
quires a stony compact texture. Consequently, whenever 
they found in a volcanic mountain sheets of stony materials 
inclined at angles of from 5° to 20° or even more than 30°, 
they thought themselves warranted in assuming that such 
locks had been originally horizontal, or very slightly in¬ 
clined, and had acquired their high inclination by subsequent 
upheaval. To such dome-shaped mountains with a cavity 
in the middle, and with the inclined beds having what was 
(*alled a quaquaversal dip or a slope outward on all sides, 
they gave the name of “ Elevation craters.” 
As the late Leopold von Buch, the author of this theory, 
had selected the Isle of Palma, one of the Canaries, as a typ¬ 
ical illustration of this form of volcanic mountain, I visited 
that island in 1854,in company with my friend Mr. Hartung, 
and I satisfied myself that it owes its origin to a series of 
eruptions of the same nature as those which formed the mi¬ 
nor cones, already alluded to. In some of the more ancient 
or Miocene volcanic mountains, such as Mont Dor and Cantal 
in central France, the mode of origin by upheaval as above 
described is attributed to those dome-shaped masses, wheth¬ 
er they possess or not a great central cavity, as in Palma. 
Where this cavity is present, it has probably been due to 
one or more great explosions similar to that which destroyed 
a great part of ancient Vesuvius in the time of Pliny. Simi¬ 
lar paroxysmal catastrophes have caused in historical times 
