Classifications of Rocks. 4& 



have been supposed to distinguish the secondary class are defec- 

 tive. But, to passoverother objections, it is also deficient even 

 as an artificial arrangement, and in points that would admit of 

 a very easy remedy, by a slight alteration. Both classes, and 

 the secondary more particularly, comprise rocks that ought to 

 be separated, and which might be so without difficulty ; such 

 as the unstratified substances, the partial deposits which have, 

 by some, been called tertiary, the alluvial substances, and the 

 volcanic rocks. The remedy for all these defects is easily 

 found, by merely substituting certain modifications, which per- 

 mit the general division into two classes to remain as it did 

 before. 



In this amended classification, the term primary is substi- 

 tuted for primitive, because it implies no theoretical conside- 

 rations respecting the origin of rocks. The classes, otherwise, 

 remain the same, except that they are first subdivided into 

 unstratified and stratified rocks; a distinction, of which the 

 propriety, and even the necessity, must be apparent to those 

 who have read the various papers and works which I have on 

 different occasions written, and the arguments from which I 

 cannot here occupy room in re-stating. 



The primary class, therefore, in this arrangement, as far as 

 the stratified rocks are concerned, is defined as it was before. 



All those unstratified rocks are also placed in this class, 

 which are found to lie beneath the whole or any one of these 

 strata, or even merely beneath the secondary, provided they 

 do not, in any case, also lie above any one secondary stratum. 

 They will not be taken out of this class though they should be 

 found intruding, in the form of veins, into the primary strata ; 

 unless these veins are connected with secondary masses, or 

 may be inferred to have proceeded from such ; but they would 

 no longer appertain to it should they continuously intrude 

 among the secondary rocks. 



In practice the distinction thus becomes geologically easy, 

 whenever the two classes are found together. But if the 

 primary class is found alone, as there is then no opportunity 

 of such a comparison, it is necessary to be guided by the mi- 

 neral characters of the rocks, and by such other circumstances 

 as experience has shown to be peculiar to them. This defect, 



