SIXTH REPORT OF THE DIRECTOR I909 I33 



mately similar in geographic distribution. There was too much 

 oscillation for that. Indeed, it is only here and there within the 

 great Appalachian and Cordilleran troughs that anything like a 

 complete sedimentary record of any one of the systems is found. 

 But these inequalities of distribution are an aid rather than a 

 hindrance in the recognition of the boundaries, because they 

 make them correspondingly more distinct where the breaks in the 

 record are expanded. 



The faunal distinctions marking the revised systems also are 

 more definite and more readily apparent than are those hitherto 

 relied on in discriminating between the Cambric and the Ordo- 

 vicic. 



In the trilobites and the brachiopods we use specific rather than 

 generic types in discriminating between the Upper Cambric and 

 the Ozarkic; likewise among the previously established conical 

 and involute gastropods. The correlation value and use of such 

 long-lived types is precisely as in the case of genera common to 

 two or more of the later systems in whose discrimination, more- 

 over, specific differences comprise a greater and greater propor- 

 tion of the competent organic data. The surviving Cambric trilo- 

 bites and brachiopods have then essentially the same significance 

 in stratigraphic taxonomy that we accord to Spirifer and Atrypa, 

 which are well developed in the Siluric and continued their exist- 

 ence into subsequent periods ; or to genera of Ordovicic trilobites 

 that are nearly as well represented in the Siluric. 



To disregard the probability of transgressions of generic types 

 from the earliest Paleozoic system into the next younger system 

 is to stand in the way of progress in stratigraphic correlation and 

 classification. The principle is recognized in the discrimination of 

 all the later systems, why not also in the case of the American 

 Cambric? The past practice of classifying, often without regard 

 to stratigraphic evidence, all formations as Upper Cambric that are 

 younger than Middle Cambric and apparently older than beds con- 

 taining supposedly indubitable Ordovicic fossils, was possibly justi- 

 fiable, but only so long as the faunal history of a great intervening 

 mass of rocks remained to be accounted for. Now, however, since 

 this old " Calciferous " hiatus has been peopled with a large mixed 

 " Cambrian " and " Ordovician " fauna, and since we have come to 

 understand the stratigraphic relations of most of the formations 

 concerned in the inquiry, some other arrangement that will express 

 the facts is desirable. We need a vehicle that will permit us to cor- 



