470 E. O. ULEICH CORRELATION OP THE STRAND-LINE 



RECENT INSTANCES SHOWING ADVANTAGES OF THE DIASTIiOPHIC METHOD 



To illustrate, let me point out a few of many instances in which the 

 application of diastrophic criteria and principles has resulted in manifest 

 improvements. So long as Ave relied solely on what we believed regarding 

 the range of certain generic types of fossils, it was practically impossible 

 to draw a satisfactory boundary between the Lower and the Middle Cam- 

 brian in the Appalachian Valley. Admitting, as we now do, that some 

 of the late Lower Cambrian types continued their existence in slightly 

 modified forms into the succeeding epoch, the two series are now sharply 

 delimitable. The case of the Cambrian and the Ordovician was even 

 worse. 



Commonly the boundary between these two systems was left undeter- 

 mined, and those who did venture to draw it seldom selected the same 

 position favored by others. Between them the boundary vacillated be- 

 tween the top of the Cambrian as now defined and the base of the St. 

 Peter. As a rule, the author's decision depended on his conception of 

 what should be called a Cambrian and what an Ordovician fauna. And 

 the decision varied greatly according to whether he relied on the evidence 

 of the trilobites or on that of some other class of fossils. In the Appa- 

 lachian Valley the lower part of the great but locally varying series of 

 "Knox v ' dolomites was stated to be of Cambrian age because it contained 

 remains of trilobites that resembled Cambrian species. The top of the 

 series, on the other hand, contained remains of mollusks that were 

 thought to place this part into the Ordovician system; but under this 

 belief several thousands of feet of intervening beds remained of undeter- 

 mined age. Moreover, subsequent investigations proved that some of the 

 mollusks referred to are much older than the supposed Cambrian trilo- 

 bites, the latter being in fact of Canadian age, while some of the former 

 are of Ozarkian age. 



We may say, then, that 10 years ago the Cambrian had no recognized 

 top and the Ordovician no bottom. Today the top of the one and the 

 bottom of the other have been definitely determined in most areas where 

 these rocks are exposed. This rapid progress is owing to the application 

 of diastrophic criteria, without which it would have been impossible to 

 show that these systems are not only sharply delimited, but that two other 

 systems of rocks belong between them. In some places deposits belonging 

 to the Canadian system were said to pass without break into the Ordo- 

 vician, but in all of these places that I have had an opportunity to study 

 a broken contact was found in the supposed "transition" beds. 



Again, relying on supposition respecting the composition of the Ordo- 

 vician fauna, we placed the boundary between this and the succeeding 



