The production of the appearances now described, belongs, without doubt, to different periods of time; and, where slips intersect one another, we can often distinguish the less from the more ancient They are all, however, of a date posterior to that at which the waving and undulated forms of the strata were acquired, as they do not carry with them any marks of the softness of the rock, but many of its complete induration. The same phenomenon which is thus exemplified on a great scale in the bowels of the earth, is often most beautifully exhibited in single specimens of stone, and is accompanied with this remarkable circumstance, that the _integrity_ of the stone is not destroyed by the shifts, whatever wounds had been made in it being healed, and the parts firmly reunited to one another.[15] [Footnote 15: NOTE XI.] 43. Though such marks of violence as have been now enumerated are common in some degree to all the strata, they abound most among the primary, and point out these as the part of our globe which has been exposed to the greatest vicissitudes. At their junction with the secondary, or where they emerge, as it were, from under the latter, phenomena occur, which mark some of those vicissitudes with astonishing precision; phenomena of which the nature was first accurately explored, and the consequences fully deduced, by the geologist whose system I am endeavouring to explain. He observed, in several instances, that where the primary schistus rises in beds almost vertical, it is covered by horizontal layers of secondary sandstone, which last are penetrated by the irregular tops of the schistus, and also involve fragments of that rock, some angular, others round and smooth, as if worn by attrition. From this he concluded, that the primary strata, after being formed at the bottom of the sea, in planes nearly horizontal, were raised, so as to become almost vertical, while they were yet covered by the ocean, and before the secondary strata had begun to be deposited on them. He also argued, that, as the fragments of the primary rock, included in the secondary, are many of them rounded and worn, the deposition of the latter must have been separated from the elevation of the former by such an interval of time, as gave room for the action of waste and decay, allowing those fragments first to be detached, and afterwards wrought into a round figure.[16] [Footnote 16: NOTE XII.]