Geology. 553 



results in interpreting ancient distributions of lands and seas, the 

 most important of these is clearly a masterly knowledge of 

 paleontology. The thirty-eight years spent by the author in 

 studying this subject have therefore formed the necessary per- 

 sonal preparation, while the growth of the several fields of geology 

 through the labors of others has supplied a wealth of literature 

 and made possible results which could not have been achieved in 

 any previous decade. 



The first 83 pages of the paper give a history of paleogeography 

 from the first maps of J. D. Dana to the present time, and a 

 statement of its methods. This is followed by a discussion of 

 the influences which enter into the shifting of the strand line. 

 A geologic history of the North American seas is also included. 

 The final 17 pages give a description of the maps which follow 

 and the new classification of geologic time suggested by them. 



The first distinctive point to be noted in regard to this work 

 consists in the large number of maps, fifty-seven having been 

 prepared and fifty-three published, each representing a separate 

 time interval, and the whole covering geologic time from the 

 Lower Cambrian to the present. The diagram of fluctuating sea 

 levels (pi. 101) indicates that at least this number of maps was 

 necessary in order to reveal graphically the successive important 

 movements that have occurred. All previous maps are therefore 

 too synthetic in that each covers so long a time interval that 

 significant changes of land and sea are concealed. 



The second distinctive feature lies in the importance given to 

 disconformities and the cartographic restriction of the original 

 limits of the seas in the direction of the ancient land masses to a 

 short distance beyond the farthermost determined outcrops of 

 their sediments. As is noted by the author, this method may err 

 on the side of too much restriction of the continental seas (p. 

 446). Most previous maps, both by representing a longer period 

 of time and freely extending the seas beyond the limits of the 

 sediments, have, on the other hand, tended to make the seas too 

 extensive in time as well as in space. By consistently following 

 his method the author is enabled to construct a diastrophic curve 

 (plate 101) which shows the percentages of the present area of 

 North America and of the United States which were exposed as 

 land at each succeeding stage. This is one of the most important 

 features of the paper. It brings out the diastrophic basis for 

 grouping the geologic periods into eras delimited by revolutions, 

 during which practically the whole continent stood for a longer 

 or shorter time above the sea level. On the major rhythm are 

 superimposed the secondary diastrophic waves, each of which 

 marks a transgression and recession of the sea and comprises a 

 geologic period. Defined on this basis, the Cambrian and Ordo- 

 vician are broken up into three periods each and the Mississippian 

 into two, called by Schuchert the Mississippic and Tennesseeic. 

 The first of these is not, in North America at least, diastrophically 

 distinct from the Devonic, the change being marked faunally 



