458 E. 0. ULRICH CORRELATION OF THE STRAND-LINE 



in the standard time scale on the general aspect of the fauna of the bed 

 in question; another considers the introduction of new faunal types, or 

 the mere presence of one or more supposedly characteristic species, as 

 surer, or at least more definite, indications of a particular time; a third 

 considers both of these methods, but inquires also into the origin and 

 migration of the faunas and floras, and is finally guided, especially in 

 drawing boundaries, chiefly by physical criteria indicating breaks in the 

 process of sedimentation. The method of the first is that of simple 

 matching of faunas and floras. The second also draws his conclusions by 

 "matching," but in his case the process is complicated by discrimination 

 and weighing of factors. Both, however, depend either wholly or mainly 

 on strictly organic criteria. The third follows the diastrophic method, 

 which, as we shall see, is the most comprehensive and scientific of the 

 three, and consequently leads to more definite results. All three of these 

 methods are being followed by paleontologists today, but happily the first 

 is no longer regarded as sufficient by those who have acquired a practical 

 field knowledge of stratigraphy. 



There is also little uniformity in the practice of geologists regarding 

 the age assignment of clastic deposits connected with a break in geologic 

 history. The break may be widely or generally determinable, and thus 

 of high significance in stratigraphic taxonomy, or it may be but locally 

 expressed and correspondingly insignificant. In either case the physical 

 evidence indicating the break may or may not be clearly discernible by 

 the average geologist, but its presence is always suggested by some change 

 in the faunas, and its plane is usually exactly determinable by those 

 trained in such investigations. Deposits of this kind have been inter- 

 preted by some as the closing facies, while others have described them 

 as introducing the succeeding age. The latter view doubtless is the more 

 correct, even when the new deposit agrees closely in lithologic character 

 with the underlying formation. 



Initial deposits closely simulating the older formation occur as a rule 

 when the latter is a sandstone whose top in the meantime has been sub- 

 jected to subaerial conditions. Commonly it is easy enough to distinguish 

 the reworked sand of the new deposit from the old, but when the simu- 

 lating material constitutes not only the basal deposit of the superposed 

 formation, but is succeeded by similar sandstone to its top, so that we 

 have two sandstone formations in contact with a known or unknown 

 break between them, the task is not so easy. But it is never hopeless, and 

 no more difficult if the hiatus is small than when it spans two or three 

 geological periods. 



