Geology. 555 



currents, but the existence of land barriers must be postulated to 

 separate them. Furthermore he holds that wide and shallow 

 seas could not be broadly subject to bottom scour in the way now 

 observed in limited localities, but would record their presence by 

 means of sediments. These two principles as rules of very 

 general though of course not universal or rigid application seem 

 well taken and give definiteness to the problem. In regard, how- 

 ever, to the valuation of the disconformities some departure of 

 view may be held from that which the author entertains. He 

 states : 



"Neither can it be admitted that . . . extensive sheets of 

 limestone have suffered erosion. If the latter were true, outliers 

 of these missing horizons would be found, for the land was so 

 low that the wearing away could not have removed them com- 

 pletely over hundreds of miles of extent " (p. 442). 

 That is, the disconformity is valued as entirely a land interval 

 rather than as partly a sea interval, A, followed by a land inter- 

 val, B, during which the sediments of time, A, are removed. The 

 reviewer would consider that A instead of being of negligible 

 value may comprise almost any part of the entire value A + B of 

 the disconformity. Blackwelder has well discussed this idea in 

 the "Valuation of Unconformities"* and Gilbert has clearly 

 called attention to the fact that we are ignorant of the maximum 

 extent of the great transgressions and still more ignorant of the 

 former maximum areas of lands, since the landward sediments 

 are immediately subjected to erosion upon the least retreat of the 

 sea and continue to be eroded through all later time. The sea- 

 ward limits of the greater unconformities are, on the other hand, 

 forever concealed. f This paper of Gilbert's, so far as the reviewer 

 is aware, is the first in American literature to call in question the 

 general conception prevailing until recent years of the continu- 

 ous reign of the Paleozoic continental seas. The questions raised 

 by Gilbert are strikingly answered in the affirmative through the 

 investigations of the past decade. The exceptional preservation 

 of outliers of the great transgressions hundreds of miles from the 

 nearest regular outcrops, in some places by downfaulting, in 

 others by fossiliferous fragments having fallen down fissures and 

 so escaping erosion, indicates how accidental is our knowledge of 

 the supposed limits of the great transgressions. Limestones 

 furthermore being subject to attack by solution are the most 

 reducible of formations in a humid climate, as indicated by the 

 broad limestone valleys rapidly developed at baselevel. Conse- 

 quently under a suitable climate it is to be expected that out- 

 lying sheets left above baselevel might be completely dissolved 

 away without leaving a trace of their former existence. The 

 theory of probabilities would indicate that the plane surface of 

 disconformities should more often be due to unlimited though 

 slight emergence of the land with following subaerial planation, 



-Blackwelder, Jour. Geol., vol. xvii, pp. 289-299, 1909. 



f Continental Problems, Bull. Geol. Soc. Arner., vol. iv, pp. 178-189, 1892. 



