RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 269 



sometimes in different parts of the same formation having varied in 

 condition from that of impalpable fineness to that of sand and gravel. 

 They are composed of calcareous, argillaceous, or siliceous materials 

 which in process of time have become more or less completely hardened 

 into rock and which in their separate condition are limestones, sand- 

 stones, and shales, respectively. These materials may, and often do, 

 occur thus separately, not only in different formations, but in different 

 parts of the same formation. Besides this, the different materials are 

 often mechanically commingled, producing rocks of a mixed character; 

 and sometimes the character of all kinds of stratified rocks is found to 

 have been materially changed by metamorphism. 



Formations differ greatly in thickness because the rate of accumu- 

 lation of sediments was necessarily very variable and because their 

 upper and lower limitation was coincident with, and due to, accidental 

 changes of physical conditions which occurred at irregular intervals of 

 time. That is, their limitation was caused by such movements of the 

 earth's crust, including both land and sea bottom, as produced a more 

 or less complete interruption of sedimentation, or change in the char- 

 acter of the same, and consequently a more or less complete extinction, 

 or geographical transference, of the life that existed in the water in 

 which the sediments were deposited. This is an almost invariable rule, 

 but in rare cases the faunal and physical delimitations were not fully 

 coincident. In such cases an abrupt faunal change has occurred within 

 the vertical range, or at the upper limit, of a formation where sedimen- 

 tation seems to have been unbroken between it and the next succeed- 

 ing one. Such cases plainly resulted from a change in the character of 

 the water as a faunal habitat, which was uot accompanied by a corre- 

 sponding arrest of, or even by a material change in, the character of 

 the sedimentation.* 



Formations may sometimes be continuously traced by the eye for 

 considerable distances, especially if the debris of erosion has been well 

 removed by denudation. In such cases no question can be raised as to 

 their identity. Sometimes also a formation may be satisfactorily rec- 

 ognized at separate but not distant localities by means of its litholog- 

 ical characteristics alone when actual continuity is obscured or hidden 

 from view by succeeding formations or other overlying material. In 

 their greater geographical extension, however, formations undergo such 

 changes of lithological character, and they often so closely simulate 

 some one or more associated formations, that their lithological identifi- 

 cation is uncertain or impracticable. Therefore, with the minor excep- 

 tions mentioned, the only known meaus by which a formation may be 

 certainly identified at any other than its originally discovered locality 

 is that which is afforded by its contained fossil remains. 



"•Reference is here made particularly to conditions that arc observable among the 

 Upper Cretaceous formations of the interior portion of North America. 



