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Tr 
230 DYNAMICAL GEOLOGY.  [Boox IL 
warm to allow the hand to be held in them, and three years later it 
was still steaming abundantly. Hoffmann records that from the lava 
which flowed from Etna in 1787 steam was still issuing in 1830. 
Yet more remarkable is the case of Jorullo,in Mexico, which sent out 
lava in 1759. Twenty-one years later a cigar could be lighted at its 
fissures: after 44 years it was still visibly steaming; and even in 
1846, that is, after 87 years of cooling, two vapour columns were still 
rising from it.* 
This extremely slow rate of cooling has justly been regarded as a 
point of high geological significance in regard to the secular cooling 
and probable internal temperature of our globe. Some geologists 
have argued indeed that, if so comparatively small a portion of 
molten matter as a lava-stream can maintain a high temperature 
under a thin, cold crust for so many years, we may, from analogy, 
feel little hesitation in believing that the enormously vaster mass of. 
the globe may, beneath a relatively thin crust, still continue in a 
molten condition within. More legitimate deduetions, however, 
might be drawn from more accurate and precise measurements of 
the rate of loss of heat, and of its variations in different lava-streams. 
Sir William Thomson, for instance, has suggested that, by measuring 
the temperature of intrusive masses of igneous rock in coal-workings 
and elsewhere, and comparing it with that of other non-volcanic 
rocks in the same regions, we might obtain data for calculating the 
time which has elapsed since these igneous sheets were erupted 
(ante, p. 46). 
Effects of lava-streams on superficial waters and 
topography.—In its descent a stream of lava may reach a water- 
course, and, by throwing itself as an embankment aeross the stream, 
may pond back the water and form a lake. Such is the origin 
of the picturesque Lake Aidat in Auvergne. Or the molten current 
may usurp the channel of the stream, and completely bury the 
whole valley, as has happened again and again among the vast 
lava-fields of Iceland. Jew changes in physiography are so rapid 
and so enduring as this. ‘The channel which has required, doubtless, 
many thousands of years for the water laboriously to excavate, is 
sealed up in a few hours under 100 feet or more of stone, and 
another vastly protracted interval may elapse before this newer pile 
is similarly eroded.? | 
By suddenly overflowing a brook or pool of water, molten lava 
sometimes has its outer crust shattered to fragments by a sharp 
explosion of the generated steam, while the fluid mass within rushes 
out on all sides. ‘The lavas of Etna and Vesuvius have protruded 
into the sea. Thus a current from the latter mountain entered the 
Mediterranean at ‘Torre del Greco in 1794, and pushed its way for 
360 feet outwards, with a breadth of 1100 and a height of 15 feet. So 
' KE. Schleiden, quoted by Naumann, Geol. i. p. 160. 
* For an example of the conversion of a lava-buried river-bed into a hill-top by long- 
continued deuudation, see Quart. Journ, Geol, Soc. 1871, p. 803, 



