416 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 



