Now, I think this argument a very strong one; in fact, although simple, almost unanswerable. For no _plant_ dwells thus in a house. We have the plant itself, but nothing more; and if this be not capable, and I hold it to be incapable, of sustaining the most vehement mineralizing process in the crucible of Nature, its history must be a brief one, and excepting in the dark “lithographs” of the coal-measures, its memory must pass away. I have already allowed an exceptionable case in favour of the _conifers_, which, be it observed, nowise resemble anything portrayed in siliceous pebbles, and it is remarkable how much this class of plants predominates in the COAL. I have always been suspicious of what are called “vegetable petrifactions.” I examined those at Tivoli, near Rome, in the year 1845, and I made up my mind that they are simple _incrustations_. In like manner, many of the buildings at Pæstum are constructed with a kind of “travertine” taken from the bed of a neighbouring river, and which rapidly incrusts any solid objects submitted to the action of its waters. But the truth is, the vegetable pipe or “straw” remains for a while, owing to the silex which entered into its composition while the plant was growing. After some years the _straw_ decays, and there is a hole or depression in that part of the pillar or pediment. On the other hand, a calcareous “menstruum,” imbibing silex and iron, hardens into a substance which, like the best mortar or cement used in building, will sometimes outlast even the blocks of limestone or oolite which it was put to bind together.