W. Upham — Estimates of Geologic Time. 209 



fauna is distinctive as a whole and it pervades the greater 

 mass of the western extension of the rocks of the Catskill 

 Mountains from the upper part of the Portage in western 

 New York nearly to the Lower Carboniferous in northwestern 

 Pennsylvania. 



As the " Catskill " in its type region comprises Portage and 

 Chemung, my proposition now is to discontinue the use of 

 Catskill as a coordinate formation term and use the term Cat- 

 skill group to include the Portage and Chemung formations, 

 the latter extending to the base of the Lower Carboniferous. 

 I believe the Chemung and Portage are formations distinctly 

 separable over a wide area, but Chemung and "Catskill" as 

 formations are only separable by a lithologic distinction which 

 progressively varies several thousand feet in stratigraphic posi- 

 tion in the extension of the beds across southern New York. 



Stevenson, in the review alluded to above, points out the 

 general unity of the upper Devonian throughout the Appa- 

 lachian region, and as its marine fauna is predominately Che- 

 mung, proposes the " Chemung " as a period name to comprise 

 three epochs ; the Portage, the Chemung, and the Catskill. 

 The new light on the stratigraphic range of the formation 

 comprised under the name " Catskill " in its typical region is 

 the ground for my preference for the older term in its origi- 

 nal significance, even though its true stratigraphic limits were 

 not originally defined. 



Art. XXVI. — Estimates of Geologic Time / by 

 Warren Upham. 



[Abridged from a paper in the Bibliotheca Sacra, Jan., 1893.] 



According to Sir Archibald Geikie, in his presidential 

 address before the British Association last August,* the known 

 rates of deposition of sediments imply that for the formation 

 of all the stratified rocks of the earth's crust a duration some- 

 where between 73 millions and 680 millions of years must be 

 required. Most geologists, before specially looking into this 

 subject, would doubtless regard the lowest of these estimates 

 as a minimum of the time needed for the processes of deposi- 

 tion and of erosion revealed by their study of the rocks, and 

 for the concurrent changes of the earth's floras and faunas 

 from their beginning to the present time. But to some geolo- 

 gists, these figures seem far too small, among whom Mr. W. J. 

 McGee, in a paper read before the American Association the 



* Nature, Aug. 4, 1892, vol. xlvi, pp. 317-323. 



