Ch. Xin.] PRINCIPLES OF CLASSIFICATION. 179 



though not always 111 the exact manner there represented, because 

 some of them repose occasionally in unconformable stratification on 

 others. This mode of superposition has been already explained at 

 pao-e 59. Where it occurs it is almost invariably accompanied by a 

 great dissimilarity in the species of organic remains of the sets of 

 strata next in succession, the discordance implying a considerable 

 lapse of time which intervened between the two formations in juxta- 

 position. During the ages which elapsed, and of which no records 

 have been handed down to us in the area in question, we may sup- 

 pose a gradual change to have been going on in the state of the ani- 

 mal creation, and the same interval allowed time for a great amount 

 of movement and dislocation to have been brought about in the 

 earth's crust, so that the strata previously existing in the region 

 alluded to had been much disturbed and their edges exposed to 

 aqueous denudation before the more modern set were thrown down 

 upon them. 



Where the widest gaps appear in the sequence of organic remains, 

 as between the Permian and Triassic rocks, or between the Creta- 

 ceous and Eocene, examples of such unconformability are very fre- 

 quent. But they are also met with in some part or other of the 

 world at the junction of almost all the other principal formations, 

 and sometimes the subordinate divisions of any one of the leading 

 groups may be found lying unconformably on another subordinate 

 member of the same — the Upper, for example, on the Lower Silurian, 

 or the superior division of the Old Red Sandstone on a lower mem- 

 bci of the same, and so forth. Instances of such irregularities in the 

 mode of succession of the strata next in contact are the more intelli- 

 gible the more we extend our survey of the fossiliferous formations, 

 for we are continually bringing to light deposits of intermediate date, 

 which have to be intercalated between those previously known, and 

 which reveal to us a long series of events, of which antecedently to 

 such discoveries we had no knowledge. 



But while unconformability invariably bears testimony to a lapse 

 of unrepresented time, the conformability of two sets of strata in 

 contact by no means implies that the newer formation immediately 

 succeeded the older one. It simply implies that the ancient rocks 

 were subjected to no movements of such a nature as to tilt, bend, or 

 break them before the more modern formation was superimposed. 

 It does not show that the earth's crust was motionless in the region 

 in question, for there may have been a gradual sinking or rising, 

 extending uniformly over a large surface, and yet, during such move- 

 ment, the stratified rocks may have retained their original horizontal- 

 ity of position. There may have been a conversion of a wide area 

 from sea into land and from land into sea, and during these changes 

 of level some strata may have been slowly removed by aqueous 

 action, and after this new strata may be superimposed, differing per- 

 haps in date by thousands of years or centuries, and yet resting con- 



