1904.] HATCHER — MARINE AND NON-MARINE FORMATIONS. 349 



mum thickness. Some distance to the eastward of this meridian, 

 however, they decrease in thickness and finally disappear, passing 

 into the overlying and underlying marine beds mentioned above. 

 In the second diagram A. would represent the fresh water and 

 middle portion of the Judith river beds, B. B. the underlying and 

 overlying brackish water beds usually referred also to the Judith 

 river formation, while C. C. and D. D. inclusive would represent 

 the Bearpaw shales at the top and the Claggett formation at the 

 bottom, C C representing littoral deposits or passage beds from 

 brackish water to true marine deposits and occurring everywhere 

 throughout this region both at the top and bottom of the Judith 

 river beds. 



Having now seen how two or more formations occurring in 

 regular sequence one above the other may originate simultaneously 

 instead of pertaining to different and distinct ages, let us next con- 

 sider what bearing this will have on the relative age of different 

 horizons in any one of these formations. To the paleontologist 

 this is an important point, since in working out the phylogeny and 

 relations of fossil forms within the same formation it is often of the 

 greatest importance to know whether a given genus or species pre- 

 ceded or succeeded another, or if the two were contemporaneous. 

 Heretofore we have been inclined to consider any given stratum 

 within a formation as having been deposited near the beginning, at 

 about the middle or near the close of that time interval during 

 which the formation was laid down, according to whether it occurs 

 near the top, in the middle or near the bottom of the formation 

 and regardless of the geographical position of that point at which 

 the stratigraphical determination was made. To say that two fos- 

 sils were found fifty feet beneath the top of a given formation which 

 had not been subjected to erosion, but at a distance from each 

 other of several, perhaps many miles, has usually been regarded as 

 establishing for them an identical or very similar age. Every con- 

 sideration has been given to the stratigraphic position, none to the 

 geographic. Yet in not a few instances the latter is of equal or 

 even far greater importance than the former, and must be consid- 

 ered in determining at just what period in the time interval neces- 

 sary for the deposition of a certain formation any given stratum 

 was laid down. Furthermore, we have been accustomed to esti- 

 mate the time employed in the deposition of any given formation 

 solely by its vertical thickness, and regardless of its geographical 



