44 Dr. Mac Culloch on 



such as it is, must inevitably occur, whatever arrangement 

 may be adopted ; as it is not usual, nor even very common, 

 for both classes to be found in contact, or at one point. 



The secondary class commences as in the ancient arrange- 

 ment, where the primary ends; but as that proceeds down- 

 wards, this extends upwards. A conglomerate and sandstone 

 frequently, but not necessarily red, is, as already observed, its 

 lowest possible boundary. But as this may be absurd, it is not 

 a necessary one ; and that must then be sought in the lowest 

 stratum or rock, which is known to be, on ordinary occasions, 

 superior to it, and of which the mineral characters, as in the 

 case of the primary strata, are known. No rules for the 

 boundary of this class must be drawn from the nature of the 

 primary rock on which its lowest member reposes ; because 

 this may be the lowest as well as the highest of that class. 

 Such also are the occasional deficiencies in the secondary class, 

 that its upper members may rest on the primary, and even on 

 the lowest of these. Thus the extremes of both classes might 

 meet ; an instance approaching to which has been elsewhere 

 quoted by me as occurring in Sutherland. With respect to 

 the unstratified substances, every such rock is considered as 

 belonging to it which is not included in the primary division. 

 But those of this class differ in one important point from the 

 corresponding ones in the other ; as they may, and do, send 

 veins through the primary rocks, without forfeiting their class. 

 It needs only be added, that their masses are generally supe- 

 rior to the strata with which they are associated. 



Such was the arrangement adopted in the work on Rocks, 

 for the purpose of describing their characters ; and as such, it 

 was suffered to remain, in other points, irregular and incom- 

 plete. As the rocks of the tertiary strata, for example, agreed 

 in character with corresponding ones in the secondary class, no 

 attempt at a third distinction was made. In the same way as 

 jasper and some other substances occur in both classes, it was 

 held unnecessary to repeal them in both, where one discussion 

 would serve, so that they were placed in an appendix, as being 

 merely accidental modifications of some members in both 

 classes. Such an arrangement, it is evident, made no preten- 

 sions to a classification, in the proper sense of the term. With 



