ORIGIN OF THE TBAP ROCKS. 93 
Saxony, wrote a book in defence of the doctrines of Werner, but 
after visiting the mountains of Auvergne, he was satisfied that 
the basaltic formations of that country are the products of volca- 
noes, and disposed to generalize the proposition, and refer all 
rocks of the same kind to the same origin. 
4. When beds of limestone, or coal, are traversed by dykes of 
basalt, or lie adjacent to a body of that substance, they are often 
much altered at, and near, the point of contact ; the limestone 
having been made to assume a crystalline structure, and the coal 
deprived of its bitumen, and charred, and we know of no way 
in which contiguous strata could produce this effect, except by 
the intense heat of that which has changed the condition of the 
other. 
The reasonings of the Wernerians were generally presented 
under the form of objections to the doctrines of their opponents, 
rather than as positive arguments in favor of their own opinions. 
Thus in the case of the basalts of Saxony: mines have been run 
quite under them, and it has been thus ascertained that they can- 
not have been thrown out of the bowels of the mountains on 
which they stand. The inference was drawn that they can- 
not have been produced by the action of fire : but whether we 
call in one or the other element, fire or water, to aid us in the 
formation of these rocks, their position is equally embarrassing. 
The great extent of some formations of basalt, was proposed 
as an objection to the opinion that this rock is of igneous origin. 
But when it is considered that the bed of lava which flowed from 
Mount Heckla in 1784, is ninety-four miles in length, fifteen in 
breadth, and in some places from eighty to an hundred feet in 
thickness, the magnitude of the largest trap formation cannot be 
regarded as presenting any very considerable difficulty. The 
last century furnished another example of the energy with which 
volcanic action may be exerted without convulsing the earth to 
any great extent. On the night between the 28th and 29th of 
September, 1759, a tract of country four miles square, in the in- 
tendancy of Valladolid in Mexico, which had formerly been cul- 
tivated ground, was thrown up to an elevation, at the highest 
point, of about 1500 feet, (the height of the Pilot Mountain ahove 
the surrounding country very nearly,) and converted into a vol- 
cano, and yet this event was unknown to men of science until 
Humboldt visited Mexico at the commencement of the present 
century. 
It was further urged by the Wernerians that basalt sometimes 
traverses or touches, strata of coal and limestone, without producing 
any change in those substances at the point of contact, and that 
it also embraces animal and vegetable remains. These are con- 
siderations of real weight, but the inference drawn from them 
was^rejected on the ground that the examples cited were few in 
number, and that the facts had in these cases been either misappre- 
hended or misinterpreted. The belief is universal amongst geologists 
9 
