Classifications of Rocks. 45 



equal neglect of geological classification, the remaining sub- 

 stances, comprising volcanic rocks, coal, alluvia, and other 

 matters, were placed in another appendix, because it would 

 have been impossible to have followed a geological order in 

 the descriptions of these, without the greatest inconvenience. 



These deficiencies, however, being palpable and acknow- 

 ledged, were of little moment ; particularly as the arrangement 

 was not intended to form part of a system of geological 

 science. The most serious defect was one, which, I fear, is 

 still irremediable. It is that which relates to the overlying 

 rocks ; a division, the very name of which is objectionable. 

 There can be no doubt that some of the porphyries and clay- 

 stones are truly primary ; but the difficulty was then, as it is 

 now, to determine which were such, and to find a mode of 

 distinguishing them from secondary ones. Thus the whole 

 were placed in one family, and in the secondary class ; a plan 

 which had no other merit but its convenience for the purpose 

 of describing the several kinds with the least repetition or 

 confusion. 



There are some practical defects, I must now add, in a clas- 

 sification of rocks, -were it even a perfect one, inherent in the 

 very nature of the subject, and consequently irremediable. 

 One of the most obvious of these, is the repetition of the same 

 substance in different classes: of which limestone, jasper, 

 siliceous schist, and others, are examples. Custom has led us 

 to treat of the limestones of. the two classes separately ; yet to 

 attempt to do the same for the other substances would not now 

 be tolerated ; besides which, there is perhaps not enough to be 

 said respecting them, to justify such a division. In the same 

 manner limestones, sandstones, and other substances, occur in 

 the secondary class, and also in what may be called a tertiary 

 one. The former, indeed, is even an alluvial rock, in the 

 shape of travertino and coral sandstone ; so that it occurs in 

 four distinct parts of the system. But I need not enumerate 

 more of these defects and difficulties. They cannot be reme- 

 died ; but we must recollect that they are in a great measure 

 artificial evils, and of our own creating. They have arisen 

 from one of those abuses of logic, which attempts to reduce 

 every thing to one system; from a desire to carry into a 



