46 . Natural History of Volcanos and Earthquakes. 
casionally been witnessed, may be satisfactorily explained by the 
rising of steam and gases, which may have the effect either of 
heating the water, or of throwing out solid bodies.* The same 
may be said of the concussions of the earth which take place, 
sometimes in horizontal undulations, sometimes in vertical shocks, 
and sometimes with a vibratory motion, backwards and forwards. 
The latter of these convulsions, called by the Neapolitans, moto 
vorticoso, is most common during the greatest earthquakes. 
Von Humboldt has proved, by abundant examples, that the 
propagation of earthquakes is not confined to any particular rock, 
but that the most varied formations are equally favorable to it. 
We infer, therefore, that the seat of earthquakes must be below 
all known rocks. Although all the rocks may be agitated, yet 
the manner of extension of the shocks in them is different, ac- 
cording to their particular quality. The earthquakes, which 
the 21st October 1766, which totally destroyed the city of Cumana, the earth 
opened at several places in the province, and vomited sulphureous water. These 
eruptions were particularly numerous in a plain, which extends towards Casanay 
o geographical miles eastward of Cariaco, and which is known by the name of 
the hollow land (tierra hueca) because it seems to be every where undermined by 
hot springs. Von picereaat il Reise, t. i, p. 482. During the violent ee 
which in one mi the city of Curaccas, on ‘the 26th March 1813, 
much water was thrown up through the cracks, that a new stream was fecabal 
At the same time the ground was also found covered with a fine ae earth, like 
volcanic ashes, which had been thrown up from fissures inthe neighborhood. The 
eruptions of volcanic masses were still more considerable during the earthquake 
consisting of volcanic matter, accumulated so as to form considerable hills, now 
alled moya. Wide rents were likewise opened es the violent earthquake in 
the north coasts of South America, last year, in order to give exit to streams of 
water which rose. It was often observed, that during the earthquakes, water 
with sand, mud, &c., was thrown up from wells, sometimes to a height of 30 ft. 
Von Humboldt relates, (Relat. Hist., t. ii, p.287,) that this phenomenon is gen- 
erally observed during the ution at Cumana. The same thing happened 
the Ist Nov. 1755 near Colares, (Philos. Trans. t. xlix, p. 416,) and also during 
the earthquake in Calabria. cn de Phys. Ixii, p. 263.) 
* Thus, during the above-mentioned ea rihangke on the north coast of South 
America oa; colu umns of smoke were seen rising out of the sea, a league from the 
shore, and in a depth of about 210 ft.; and in the night, flames were seen issuing 
m the same spot, which illuminated all the coasts of the island. After each 
shock, the sea retired, left the ships which were in the bay aground, and laid bare — 
the rocks to a great depth; the waves at the same time ran to a height of 16 ft. to 
20 ft. During the shocks the earth opened and closed again very rapidly. When 
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