On the south side of Edinburgh, I have seen, in
little more than the space of a mile from east to west, nine or ten
masses of whin-stone interjected among the strata. These masses of
whin-stone are from three or four to an hundred feet thick, running
parallel in planes inclined to the horizon, and forming with it an angle
of about twenty or thirty degrees, as may be seen at all times in the
hill of Salisbury Craggs.
[Note 14: See Mr Whitehurst's Theory of the Earth.]
Having thus described these masses, which have flowed by means of
heat among the strata of the globe, strata which had been formed by
subsidence at the bottom of the sea, it will now be proper to examine
the difference that subsists between these subterraneous lavas, as they
may be termed, and the analogous bodies which are proper lavas, in
having issued out of a volcano.[15]
[Note 15: The Chevalier de Dolomieu, in his accurate examination of
Aetna and the Lipari islands, has very well observed the distinction of
these two different species of lavas; but without seeming to know the
principle upon which this essential difference depends.
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