STRATIGRAPHIC TAXONOMY 601 



tions of the median parts of the continent, whereas the present discussion 

 refers to the continent as a whole. If the idea in general is well founded 

 rhythm in vertical oscillation is indicated in both cases. 



Accessible deposits of detrital material are common or predominant in 

 the terminal divisions of eras. Movements at these times differed from 

 those in other intersystemic intervals in that they alfected or rather ex- 

 tended to areas farther inland than did those in the next two intersys- 

 temic intervals. Ehythm is thus again indicated by periodicity in abun- 

 dance of clastic matter in marine deposits as follows : 



Abundant and coarse clastic matter Less and usually finer matter in the 

 in the 



Belt-Cambrian interval, 



Ozarkiau, Canadian, and early Ordo- 

 late Ordovician and early Silurian, vician, 



late Silurian, Devonian, Waverlyan, 

 late Tennessean and Fennsylvanian, and early Tennessean, 



Jura-Triassic, Comancbean. and early 

 late Cretaceous and early Eogenic. Cretaceous, 



late Eoi^enlc and VNeogeuic. 



The differences were well marked in the Eopaleozoic and N"eopaleozoiC; 

 but with the increasing dominance of emergent phases in the Mesozoic 

 and Cenozoic (see page 3-11) the contrast diminished to the vanishing 

 point. 



DIA8T1WPHIC RHYTHM IN RELATION TO THE DURATION OF GEOLOGIC AGES 



As the final illustration of rhythm in geological processes I shall cite 

 a few facts and suggestions bearing on the duration of geologic periods, 

 namely, rhythm is suggested by the apparent approximation in time value 

 of deposits referred to each of the Paleozoic periods. Reduced to a lime- 

 stone basis, the average aggregate thickness of accessible beds representing 

 most of these periods is between 4,300 and 6,000 feet each (see page 398). 

 The aggregate for the Silurian, Waverlyan, and Tennessean systems, so 

 far as known, falls more or less beneath this estimate, but oscillation and 

 emergence prevailed to such a degree in these periods that the determina- 

 tion of the complete sequence for each is very difficult. When the se- 

 quence shall have been worked out, it is expected that the aggregate 

 thickness for the Silurian at least, and perhaps also for the Waverlyan, 

 will be brought much nearer the average than are the present figures. 

 The Cambrian and the Devonian, as developed in the Cordilleran basin, 

 seem to exceed the average Paleozoic system in thickness. Regarding 

 these it may be suggested that the record here approximataes in complete- 

 ness that laid down within the continuously submerged oceanic basins.. 



Xr. — Bull. GKor.. Hon. Am., Vol. 22, 1010. 



