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 ti'ap ; 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 scorife 

 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 a rock. 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.f These ridges and peaks of trap present indu- 

 bitable evidence, that they are not in tlieir 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. 



t We may say continuously, in a geological sense, for, although the ranges are 

 often interrupted, they are on the whole continuous, and the subjacent sandstone is 

 strictly so, except where it is cut in two, by dykes of trap or other rocks. 



