124 W. J. McGee — Three Formations of 



Four years ago the writer undertook to ascertain the origin 

 and relations of certain conspicuous deposits in the District of 

 Columbia, under the direction and auspices of the U. S. 

 Geological Survey. It was soon found that while certain for- 

 mations of the region — es}3ecially those containing fossils, — 

 had received more or less study, while the various formations 

 had been classified by means of the fossils contained in a few 

 of them, and while the entire region had even been repeatedly 

 mapped geologically, nothing was accurately known of the 

 exact relations of the different fossiliferous and unfossiliferous 

 deposits forming the portion of the Coastal Plain within and 

 contiguous to the District of Columbia ; it was soon found, 

 moreover, that the formations in question are not only gen- 

 erally unfossiliferous in the district, but that the deposits 

 themselves are destitute of constant and definite petrographic 

 and structural characters whereby the stratigraphy might be 

 ascertained ; and thus it was early found difficult, if not impos- 

 sible, either to correlate the different local exposures and de- 

 posits among themselves, or to establish their relations to 

 formations already classified elsewhere by commonly employed 

 methods. Accordingly, the investigation was commenced de 

 novo; and in default of fossils or definite structural characters, 

 locally applicable methods, standards, and criteria were devel- 

 oped as the work progressed : Conditions of deposition were 

 inferred from .deposits, and continent-movement was in turn 

 inferred from evident conditions of deposition superinduced 

 thereby ; conditions of degradation in unsubmerged areas were 

 inferred from the topographic forms thereby developed, and, 

 since degradation is preeminently dependent on base-level, 

 another means of inferring continent-movement was thus 

 evolved ; the record of events interpreted from earth-forms 

 fashioned in accordance with determinate principles on the 

 one hand was compared with the record interpreted from cor- 

 relative deposits on the other hand, and history was thus de- 

 duced from independent but consistent and cumulative testi- 

 mony; and final correlations were made through deposits 

 regarded not only as rocks but also as indices of continent 

 movement, and at the same time through the correlative topo- 

 graphic forms. In short, the methods, standards, and criteria 

 have been of necessity physiographic rather than paleontologic 

 or petrographic. 



Certain preliminary results of the work appear of sufficient 

 moment to merit a place in the standard medium of American 

 students of science. They are summarized in the following 

 pages. 



In the present sensitive if not revolutionary condition of 

 geologic terminology, it may be wise to define, without spe- 



