41G ON THE GEOLOGICAL RELATIONS often entangle fragments of limestone ; and might thus include a fossil shell also. Such an occurrence will probably however prove rare, as it will also leave the present rule intact. If, now, the stratified rocks do not always contain organic fossils, the reasons for the exclusion will im mediately appear, in each case. Generally, these are, that some strata may have existed before the creation of organic beings, that some have undergone changes destructive to them and to their remains, that some earths are unsuitable to their habits as places of resi dence, that deposits of stony or earthy matters must often have been made in too short a time to permit of their multiplication, that even the present sea does not every where contain the living beings, and that there must have been antient situations and circumstances where they did not exist as such. Their existence in the strata is easy of explanation. The marly deposits of a lake, or an oyster bank in the sea, are the preparations for future rocky strata of organic fossils, as their powder forms compact lime stones, and their sand oolithes; the latter produced daily under our eyes. It is equally obvious that they must predominate in limestones, since they have generated these, and that they should occur in shale rather than in sandstone, because living shells do the same, or inhabit mud in preference to sand. If they are colonial in rocks, so are they in the sea; if inter mixed, we have still their living models, in a state of intermixture. The secondary marine strata are thus their principal seats; and why they should occur in the tertiary ones and the alluvia, is too obvious to require a word. If, in the days of geological igno rance, they were thought limited to the secondary strata, that time is past: though, ever unwilling to