272 KEPORT OF NATIONAL MUSEUM, 1892. 



theui, and makes it especially necessary in determining the character- 

 istics of each formation to study their respective faunas each as a whole. 

 Still, it is usually the case that the vertical range of a large proportion 

 of the species is not found to pass beyond the vertical limits of the for- 

 mation in which they occur. 



The beginning and ending of the sedimentation which produced each 

 formation having been dependent upon the unstable conditions of the 

 earth's crust, the occurrence of the displacements of which were irreg- 

 ular in time and variable in extent, formations are necessarily not only 

 very unequal as regards their geographical extension, but also as 

 regards their relative value in stratigraphical classification. For these, 

 among other causes, they are also unequal in their relative importance 

 as representing stages in biological development. 



The foregoing remarks apply especially to marine formations, and 

 they are of general applicability. The manner, however, in which 

 occurred the upper and lower delimitation of the series of fresh water 

 formations in the interior region of North America was evidently some- 

 what exceptional. These deposits took place in waters which rested above 

 ocean level, and their differentiation into formations was evidently 

 largely due to the shifting level of the waters in which they were 

 respectively laid down, as well as to the shifting of the areas of denu- 

 dation from which their sediments were derived. The latter was doubt- 

 less also the cause of the differences in lithological charactistics of 

 those formations. 



Because both the time and areal limits of marine faunas were always 

 indefinite, especially as regards both the time and geographical range 

 of certain species, it is plain that it is the fauna as a whole, and not 

 separate members of it, that must be regarded as characterizing a 

 formation, although a single species is often sufficient for its identifica- 

 tion within a limited district or region after its characterization has 

 been determined by means of its fauna, aided by its physical features. 



The remains of aquatic faunas only have been considered in connec- 

 tion with the foregoing discussions of the origin and limitation of the 

 sedimentary formations, because the life history of those faunas only 

 was intimately connected with their production. The greater part of 

 the fossiliferous formations of the earth contain no other remains than 

 those of aquatic faunas, but in many formations remains of members 

 of contemporary land faunas and floras are found commingled with 

 those of aquatic faunas. The latter were intombed where they origi- 

 nated, but the others reached their intombment by the indirect way that 

 was described in the preceding essay. It is therefore plain that the 

 remains of denizens of the waters in which a given formation was de- 

 posited are more characteristic of it than those of contemporaneous land 

 animals and plants could be, because the aquatic fauna which they rep- 

 resent, whatever may be its value as representing a stage of biological 

 development, was dependent for its existence upon the same conditions 

 which were necessary to the production Qf tue formation, 



