10 STKATIGRAPHICAL GEOLOGY 



to those caused by depression; a deep-sea area, where deposition 

 has been very slow, may by elevation be brought nearer to a coast 

 line, and beds containing a deep-sea fauna may be succeeded 

 immediately by others more rapidly accumulated, and embedding 

 a shallow-water assemblage. The change of life in such a case 

 would be great, but the lapse of time would not be correspondingly 

 long. If, however, elevation is continued till the area is raised 

 above the sea-level, and remains for a time as a land surface, then a 

 plane of erosion will be formed, and the change of fauna will be 

 accompanied by stratigraphical discontinuity. The break then may 

 be very considerable, but it may or may not coincide with the epoch of 

 change from one fauna to another in an adjacent area of subsidence. 



Notwithstanding this, it is true that the continuity of deposition 

 may outlast the duration of a certain fauna ; and it may become 

 necessary to draw a line between two faunas of systematic 

 importance in the midst of a conformable series of strata ; in such 

 a case it will naturally be drawn where the greatest palseontological 

 break occurs. In other words, a life-assemblage, characteristic of 

 one period in the world's history, may continue to inhabit any area 

 until such changes occur as to cause the introduction of an assem- 

 blage which has been gradually developed outside that area, and 

 these changes may take place with or without interruption of 

 deposition in that particular area. 



From the preceding remarks it will be seen that it is impossible 

 to define a system as a series of deposits formed during one 

 complete downward and upward movement or oscillation of some 

 one part of the earth's surface. Moreover, if we could so define a 

 system, the systems of one region could not be coeval with those of 

 other regions. Systems must therefore have a palaeontological 

 value, and a system may perhaps be defined as a consecutive 

 series of strata formed during the prevalence of certain generic 

 forms of life throughout a large part of the earth's surface. 



The student must be prepared to find that systems founded 

 on the differences in successive faunas will not include anything 

 like equal thicknesses of rock. If, as appears most probable, the 

 differentiation of species and the development of higher forms has 

 progressed in a constantly increasing ratio, it is clear that the time- 

 value of systems based on such changes will become less and less 

 as we approach modern times. It is certainly a fact that the 

 same forms of life extend through a much greater thickness of 

 rock in the earlier than in later geological times ; and there is 

 no reason for supposing that the production of rocks went on 

 more rapidly (at any rate during later Palaeozoic periods) than at 

 the present time ; limestones must always have been of slow and 



