RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 271 



oceanic extent. These, like existing- marine waters, notwithstanding 

 their extensive continental and island shore lines, which became in 

 part original limits to formations, had world-wide continuity, but it 

 is evident to every geologist that the most extensive of the mari in- 

 formations in their separate physical and biological identity have 

 rarely exceeded a few hundred miles in extent, and they are often 

 much less. The variable physical limits to the areas of sedimentation 

 within which were produced the separately recognizable formations 

 were coincident with the indefinite intra-marine faunal barriers, just 

 mentioned, and all marine formations more or less completely merged 

 both their physical and faunal identity into that of those which were de- 

 posited in adjacent waters. It is to be inferred that climatic influences, 

 or rather those of temperature, had much to do with faunal limitation, 

 but temperature was doubtless in large part equalized by the currents 

 which conveyed the sediments that produced the formations. 



The geographical definition of marine sedimentary and faunal areas. 

 and consequently that of the resulting formations, was mainly or wholly 

 due to the various and shifting conditions of land and sea bottom, 

 which in different parts of the world and during successive geological 

 epochs modified or changed the distribution of sediments within the 

 great areas into which the oceanic waters were thus divided, and it was 

 generally accomplished without affecting the aqueous continuity 

 between them. In a large proportion of cases these shifting conditions 

 did not affect the continuity of those waters, and they were sometimes 

 so slight as to leave the resulting formation with illy defined upper and 

 lower limitations, as well as with their usual indefinite geographical 

 boundaries. They were often so great, however, as to elevate and long 

 retain the former ocean bottom above the water level, and to thus pro- 

 duce a greater or less unconformity of, or a longer or shorter time hiatus 

 between formations. • 



While shitting conditions of sea bottom constituted the principal 

 factor in limiting areas of sedimentation, they sometimes caused the 

 partial overlapping of the borders of contemporary formations by 

 having alternately shifted those of adjacent sedimentary areas, thus 

 adding to the usual iudefiniteuess of such boundaries. 



The upper and lower limits of formations we e sometimes produced 

 by the elevation of sea bottom above water level and its resubmergence, 

 in which case those limits were sharply defined. In other cases the 

 movements of elevation and depression were too slight to entirely 

 interrupt sedimentation, and those or other physical changes were too 

 slight to prevent the survival of certain members of the earlier fauna 

 as members of the later one. Indeed, it is through such survivals that 

 continuity of life has been preserved during the whole range of geo- 

 logical time. In such cases the physical difference between the forma- 

 tions is usually slight. This, added to the partial commingling of their 

 faunas, sometimes renders it difficult to fix upon a dividing line between 



