300 Keyes — Significance of Certain Unconformities. 



in this brief statement, the relative values of the biotic meth- 

 ods and a purely physical or orotaxial method have been 

 strongly contrasted. For half a century the fossils of the 

 region have been studied and the most that has come out of it 

 has been only the geological age determined in the most gen- 

 eral way. For the rock- succession few sharp lines have been 

 developed and drawn by this means. 



On the whole, recourse of late has been had only to the phys- 

 ical breaks in sedimentation. These bounding planes have 

 given in the field both criteria for formational separation and 

 precise data for quantitative correlation. The former have 

 been surprisingly practical in their application. The latter 

 are particularly instructive on account of furnishing many 

 instances in which dependence upon fossils has been largely 

 done away with. In fact, in the working out of the general 

 scheme, aid from the contained organic remains of the different 

 geological formations has been received only in a most general 

 way, and frequently only as an independent check, among sev- 

 eral other distinct methods. The paleontological conclusions 

 usually lag so far behind the deductions derived from other 

 sources in the field that in this as in other numberless cases 

 they are almost useless as practical aids. They come in after 

 the important questions have been already conclusively settled. 



In several instances fossils have had to be neglected alto- 

 gether, since, the evidence afforded by them was so indecisive. 

 Some of these cases present similar conditions to those 

 described by McGee for the middle Atlantic slope, where he 

 stated that "during the past two or three years more has been 

 learned of the stratigraphy of the formations through physical 

 means than through biotic methods in the previous 50 years." 



One of the most instructive facts brought out by the present 

 physical scheme has been the manifest necessity of complete 

 revision of many conclusions arrived at through purely paleon- 

 tological means. In such regions as the Rocky Mountains, 

 where the rock formations are exposed on such a much grander 

 scale than in more humid parts of the country, the fossils can 

 never have the importance which they have in the last men- 

 tioned areas ; in fact, their use may be largely done away with, 

 and in actual practice they are very generally neglected. This 

 fact the paleontologists have failed to take cognizance of, at 

 least as far as published accounts indicate. Attention has been 

 also called to this fact by Emmons, King, and others who have 

 worked in the Rocky Mountain field, but who have not had to 

 depend wholly upon the fossils in their stratigraphic efforts. 



In the periods of mountain-building and in orogenic move- 

 ment a quantitative measure of a regional geologic record is 

 believed to be found. 



New Mexico School of Mines, 

 Socorro, New Mexico. 



