128 Igneous Origin of some Trap Rocks. 
cause the trap was deposited after the sandstone, and the effects are 
common to both rocks, although most conspicuous as regards infla- 
tion in the trap; but as regards induration and change of color, they 
are most striking in the sandstone ; the effects on both rocks are just 
such as, from their nature, we must expect from intense heat acting 
under great pressure. 
It is well known, that in deep currents of lava, the surface, under 
no pressure but that of the atmosphere, is usually covered with scori# 
and slag, resembling the ordinary rejections of furnaces, and that near 
the upper surface, the lava is often extremely inflated and vesicular, 
and that these appearances, although existing often below, do on the 
whole decline, and finally vanish, so that the mass at a certain depth be- 
comes like arock. Such portions would never be suspected to be of 
volcanic origin, except by a person familiar with such appearances, and 
it is this passage, on the one hand, of the undoubted lava currents into 
rocks which cannot, in many instances, be distinguished from basalt 
and other members of the trap family ; and on the other hand, of 
trap rocks into the porous and vesicular strata, and into other forms 
which can scarcely be distinguished from the same varieties of lava; 
it is this double approximation which, among many other considera- 
tions, gives such strong countenance to the igneous origin of trap 
rocks. The subject is a great one, and has often occupied the at- 
tention of able men.* We cannot now pursue it any farther, than 
to apply it to the case of the Hartford Quarry. No theory can, in 
this case be admitted, which does not embrace also the vast formation 
of which this ridge is a member ; or at least the contiguous chains, 
some of which, within a few miles of this place, present mural fronts 
composed of columns of several hundred feet in elevation, and form a 
part of trap ranges that run almost continuously, for one hundred 
and twenty miles.t These ridges and peaks of trap present indu- 
bitable evidence, that they are not in their original condition. Their — 
slopes are covered with their ruins, and these in enormous quantity, 
are often scattered over the plains and valleys. It seems fair to infer, 
that the present surface is not the original one, and that these trap 
ranges were formed under superincumbent masses, which have been 
* See President Cooper’s lecture, Vol. IV. of this Journal. 
aot We may say continuously, in a geological sense, for, although the ranges are 
often pted, they are on the whole continuous, and the subjacent sandstone is 
strictly so, except where itis cut in two, by dykes of trap or other rocks. 
