PROGRESSIVE OVERLAP 613 



be no escape from it — the name Ohio shale, adopted for a black shale 

 of Upper Devonic age, which was probably deposited at the mouth of 

 a great river, is not applicable to the Black shale of the southern Ap- 

 palachians ; but the name Chattanooga shale may be applied, if it is dis- 

 sociated from the idea of any definite age relations. 



It may be recalled in conclusion that stratigraphers have not hesitated 

 to consider the base of the Black shale as rising in the scale through the 

 Devonic, but they have been reluctant to carry it higher than that horizon. 

 Williams alone of recent wxaters has suggested that the Black shale did 

 continue on into the Mississippian ; but he, also, has considered the basal 

 portion in all exposures as Devonic. The evidence to the contrary is, ' 

 however, so overwhelming, and the explanation here set forth accounts 

 so perfectly for all the observed phenomenon, that the old assumption 

 of the synchroneity of the different parts of this formation can no longer 

 stand, since it has no basis in fact. 



Statement of the Principle of the Eegressive Overlap 



This term is applied to the arrangement of strata produced by a re- 

 treating sea, the result either of a progressive elevation of the sea bottom 

 or of stationary conditions with a continued supply of detritus. A slow 

 rate of subsidence of the sea bottom, with an excessive supply of detritus, 

 such as might result through a change in the climate from dry to moist, 

 would have essentially similar results. 



A slowly retreating sea will carry the shore zone seaward — that is, in 

 the direction of retreat. As a result, the shore detritus will be carried 

 farther out with reference to the original position of the seashore. In 

 other words, the various belts of shore-derived detritus will migrate in 

 the direction, of shore retreat and at approximately the same rate. The 

 migrating belts of shore detritus will thus pass successively over areas 

 of formerly deeper water, and hence over areas of offshore deposition. 

 If the retreat' is a gradual one, the upward gradation from offshore to 

 nearshore deposits, or in general from fine to coarse deposits, will be a 

 gradual one. In any case, however, the result will be the formation of 

 a conglomerate or sandstone of emergence* or a retreatal conglomerate or 

 sandstone bed. Since, however, during the retreat offshore deposits are 

 continually forming at a distance from shore, we may consider that in 



* A. Rutot : Les phenoraene de la Sedimentation marine, Bull, du Musee Royal d'Hist. 

 Nat. d. Belgique, ii, p. 41, 1883. I am indebted to Dr A. C. Lane for calling my atten- 

 tion to this author, who has treated some of the principles here discussed. The refer- 

 ence came too late to be made use of in the body of the paper. Reference should also 

 be made to Dr A. W. G. Wilson's paper, in Can. Rec. Sci., July, 1903, vol. 9, no. 2, this 

 author also recognizing the bearing of these principles. 



