DISCONFORMITY 823 



is exceeded by deposition at B, then the hiatus at A will be smaller 

 than twice the depositional equivalent by a corresponding amount. 

 When the amount of erosion is zero at A, the hiatus will be repre- 

 sented by the deposit at B above, when formation d will rest discon- 

 formably on h at A, instead of resting on a. (Fig. 200, IV.) 



The disconformity at A may be scarcely indicated in the strata, 

 and, if it were not for the fact that formations elsewhere found 

 intercalated between the two strata (as at B) are missing here, the 

 disconformity would not be recognized at all. Very many such dis- 

 conformities exist in our formations, but few of them are readily 

 recognized on account of the parallelism of the strata. Sometimes 

 an erosion interval and the subsequent encroachment of the sea are 

 indicated by the existence of a basal conglomerate in the later 

 formation, which includes fragments of the earlier one. In some 

 cases, where a rapid transgression of the sea took place, or where 

 fine, residual soil on the old surface is but slightly reworked, such 

 basal rudytes may be wanting; and the two formations follow ap- 

 parently with perfect conformity upon each other. This is the 

 case with the black Chattanooga shale, where it rests upon the 

 Rockwood clays in the Appalachians. The hiatus here comprises 

 the whole of the Devonic and part of the Siluric as well, yet the 

 contact, though abrupt, appears like a conformable one. Careful 

 examination should, however, reveal in such cases a more or less im- 

 perfect upper surface of the lower bed, where some traces of ero- 

 sion are still visible. When this upper surface is an undoubted 

 deposition surface, i. e., when it shows no traces of erosion what- 

 ever, and when furthermore the succeeding beds show no evidence 

 of derivation from the underlying bed, the existence of a discon- 

 formity may be doubted. 



Examples of contacts where disconformities have been assumed, 

 but are not supported by evidence of erosion, are the contact be- 

 tween the pyritiferous Brayman shale and basal Siluric sandstone 

 (Binnewater) in the Schoharie region, and the Oriskany-Esopus 

 contact in the Helderbergs of eastern New York. In the former 

 case the Brayman shales are known to be of Salina age? while the 

 sandstones on which they rested were regarded as of Lorraine age. 

 There is, however, no evidence of a hiatus comprising most of the 

 Siluric between these two formations, the basal arenyte being too 

 intimately related to the shale and of the same age. The hiatus, 

 known to exist in this region, occurs at a lower level, unless, in- 

 deed, as has recently been suggested by Ruedemann and others, the 

 Brayman shale is Upper Ordovicic. The Oriskany-Esopus contact 

 of the Helderbergs also has the aspect of a conformable one. This 



