74 
devoted to the history of earthquakes and volcanoes, the follow- 
ing remarkable statements. In a district of Mexico, between 
two streams called Cuitimba and San Pedro, suddenly, on the 
28th of September, 1759, a tract of ground from three to four 
square miles in extent, rose up in the form of a bladder, to a 
height of upwards of 500 feet. Flames broke forth over a 
surface of more than half a square league, and the ground, as 
if softened by heat, could be seen swelling and sinking like an 
agitated sea. Vast rents opened in the earth, into which the 
two rivers precipitated themselves, reappearing afterwards at 
some distance from among little volcanic cones, called hornitos, 
which sprang in great numbers out of an immense torrent of 
boiling mud, with which the whole plain became covered. “ But 
the most astonishing part of the whole phenomena was the open- 
ing of a chasm vomiting out fire, and red-hot stones and ashes, 
which accumulated so as to form a range of six large mountain 
masses, one of which is upwards of 1,690 feet in height above 
the old level, and which is now known as the volcano of 
Jorullo" (43). 
Paragraph (46) contains a description by Sir Stamford 
Raffles of an eruption from Mount Tomboro, in the island of 
Sumbawa, which gave perceptible evidences of its existence to 
a distance of 1,000 miles from its centre, by tremulous motions 
and the report of explosions. “ I have seen it computed/' 
Herschel states, “ that the quantity of ashes and lava vomited 
forth in this awful eruption would have formed three mountains 
of the size of Mont Blanc" (47). 
Many other instances of upheavals and eruptions that have 
occurred in recent times might be collected from the writings of 
geologists, especially those of Lyell. It will suffice for my 
purpose to have mentioned the foregoing. I shall now only 
add that earthquakes frequently produce subsidence , as well 
as elevation, of the ground, and that there are also cases of 
subterraneous action, which are akin to that which produces 
earthquakes, but do not operate in the same fitful and violent 
manner. For instance, the northern gulfs, and borders of the 
Baltic Sea, are steadily shallowing ; and the whole mass of 
Scandinavia, including Norway, Sweden, and Lapland, is rising 
out of the sea at the average rate of about two feet per 
century" (9). 
I proceed, next, to the consideration of the nature of the forces 
by which sudden and violent changes on the earth's surface 
might be produced, with reference, for the present, only to 
changes such as those above described, which are known to 
have taken place in comparatively recent times. Respecting the 
