APPENDIX. 475 



Its thickness, changes of dimension, inclination, curva- 

 tures,, fractures, and other obvious matters, follow; 

 while I have already said, that such inclination can 

 never be determined from partial observations. Nor 

 let fissures and joints be confounded with the divisions 

 of strata, nor the concretionary laminar structure with 

 the effects of stratification. It is by such attentions 

 that the correspondence or continuity of distant strata 

 are traced; as thus also we learn to supply that which 

 cannot be seen. 



As to successions of strata, much more is required ; 

 implying the order of those, the nature of the rocks, 

 in all their mineral characters, the modes of division, 

 and, above all, the parallel groupings, and the sepa- 

 rations of such groups from others in reverse order, 

 together with the peculiarities of structure in such 

 places. Thus it is that we discover the points of re- 

 volution, and also learn to trace all the partial deposits. 

 As to inferior groups, let the geologist ever remember 

 how much of hypothesis there is, and hence be cautious 

 lest this mislead him from the truth, which, if he truly 

 seeks it, he will find. The accidents which depend on 

 the unstratified rocks must be already understood : 

 and as to any peculiar circumstances engaged in 

 the successions of strata, it is sufficient to refer to 

 the preceding chapters. Yet under the investigation 

 of great tracts, let the geologist also attempt to 

 define the limits of sets of strata, and to deter- 

 mine the circumstances under which they were de- 

 posited ; and further, how far there are correspondences 

 under obvious differences, for in this too have we been 

 egregiously misled. And lastly, let him ever recollect 

 that he has neither formed nor conveyed a clear idea 

 of his work, unless his reader can comprehend it as if 

 given in a section, or can construct one from his de- 



