Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/238

 which are supposed by many persons to demonstrate the igneous origin of trap.

About the middle of the broken face of the hill a portion of a stratum of sandstone is to be observed entangled in the mass of the rock, and at the same time much broken and bent. But there is nothing in its appearance so very different from the other instances of this fact which have been frequently described, as to call for a particular detail.

Proceeding from this point eastward another set of junctions is seen, of which I have not met with any resemblance in other places, and which offers some anomalous appearances. The stratified rock which is here conjoined with the trap, is perfectly similar to the best characterized graywacké slate, of a fine grain and greenish hue. It is easily separated into flakes, and on being broken, exhibits slender scales of mica. A remarkable change of its aspect may be seen where it approaches the trap, and before it comes into actual contact with it. If the weathered surface only be examined, it shews a series of solid laminæ alternating with, or rather graduating into other laminæ which have a spongy appearance, or abound with open cavities similar to those of weathered amygdaloids. These cavities increase in number and magnitude towards the middle of the spongy laminæ. The same appearance of pores or cavities is to be observed at most of the points of contact where the graywacké is much contorted and mixed with the trap. A good illustration of this appearance may be given by comparing it with that assumed by the burnt micaceous schist sometimes found in the walls of the vitrified forts, and which by the action of the fire on some of its laminæ is swelled up and thickened, apparently from the disengagement of aeriform matter. When, however, this stone is broken, the cavities are found to be filled by