514 GARVH ISLAND. GEOLOGY. 



of comprising, among other rocks, those which contain 

 evidences of the existence of marine animals prior to the 

 deposition of the secondary strata ; and this class, like the 

 secondary, has been generally supposed definable by some 

 boundary. Into the doubts respecting the propriety of 

 this arrangement, it is not necessary here to enter for- 

 mally ; some have been occasionally pointed out as they oc- 

 curred, which appear sufficient to prove that it is arbitrary ; 

 while as an artificial class, it does not seem conducive to 

 any practical purposes, introducing confusion where it in- 

 tends to establish regularity. This division has in fact no 

 assignable boundary towards the primary strata; since 

 that limit, if founded on the criterion above named, must 

 be removed as often as an organic substance is discovered 

 in a rock that is not secondary. Hence it has happened that 

 most of the primary strata, either from containing such 

 remains, or from being situated above rocks in which they 

 exist, have, in some situation or other, become entitled to a 

 place in this class, which thus loses the convenience even of 

 an artificial distinction. With the exception of granite, it 

 is not probable that geologists have yet discovered a rock 

 beneath which organic remains may not be found. As they 

 diminish in number, in a general sense, the further we re- 

 cede from the most recent strata, it is plain that, among the 

 lowermost rocks, they may occur so rarely as still to have 

 escaped observation ; a circumstance of which the chances 

 would be increased by their more limited variety, more 

 complete loss of texture and shape, and more simple 

 forms. Their gradual disappearance in those cases where 

 the secondary limestones assume the massive structure 

 and crystalline texture, described in the account of Sky 

 and of the Isle of Man, will illustrate this opinion, and 

 suggest the possibility that even the common primary 

 limestones may originally have contained organized bodies. 

 Perhaps when observations have been further multi- 

 plied, it may yet be ascertained, that there has been 

 no portion of time during the deposition of the strati- 



