contemporaneous and an intrusive greenstone is simply this: the former rose through a fissure until it reached the surface, and then rolled out as a flat parallel sheet; the latter may have been erupted from below at the same time, yet, owing to different circumstances, never reached the surface, but spread out among or cut through the strata underneath. And so, when we come to examine in quarries, ravines, and other exposures, the remains of two such eruptions, we soon ascertain the relative age of the former from that of the strata among which it occurs, but as to the date of the latter we are wholly at a loss, for it gives us no clue by which we can show whether it was erupted before or after the other. We can but compare the mineralogical character of the intruded with that of the contemporaneous masses in the same district, and, from the resemblance which may be traced between them, draw at the best but a doubtful inference as to their relative dates. The contemporaneous traps always assume a bedded form, the intrusive occasionally do so; and the question naturally arises here, what are the tests whereby a bed of trap may be known to be contemporaneous and not intrusive? The answer is happily a simple one. An intrusive mass is found to alter to a greater or less extent the rocks in contact with it; if it occur as a dyke, then the beds on either side have been cut through and probably otherwise affected; if it take the form of a bed or sheet, the strata lying above and below it will be found to be both altered, showing evidently that a heated mass has been interposed between them, and consequently that the igneous rock is of later date than any of the strata among which it occurs. In the case of a contemporaneous melted trap, however, the appearances presented are different; it always takes the form of a flat bed corresponding to all the inclinations and curvatures of the sandstones, shales, limestones, or other strata among which it lies. If examined carefully, it may be found not unfrequently baking and contorting the bed that forms its pavement, but producing no change whatever on that which composes its roof. It may be capped and underlaid by layers of shale, and in such a case we might not improbably find the shale below it highly baked, so as to resemble a sort of rude pottery, while the shale above would present no sign of such metamorphism, but on the contrary might display its delicate plants or shells down to the very surface of the trap, and were the latter concealed from view we should never suspect, from the aspect of this shale, that any igneous rock existed in the neighbourhood. The inference to be drawn from such appearances seems very obvious. Had the upper shale been in existence when the greenstone or basalt was erupted, it would have suffered an alteration similar to that produced on the shale below; and the fact, plain and palpable, that it has undergone no such change, shows pretty clearly