DETERMINATION OF HORIZONS BY UNCONFORMITIES. 209 



pointed out by various evidences outside of New England. It consti- 

 tutes the last and the most marked unconformity by erosion on the island 

 of Marthas Vineyard. 



Extension of the Unconformities to Block Island. 

 a vailability of unconformities in determining horizons. 



The use of unconformities in correlating horizons is permissible in the 

 New England islands, where in the Pleistocene series, fossils, even if 

 found, are at their minimum value as determinants in questions of strati- 

 graphic succession. Block island lies in the same geologic province as 

 Marthas Vineyard and duplicates in every essential structural feature 

 the geology of that island. 



There is one case in alternating unconformity and deposition in which 

 unconformity alone proves insufficient for the purpose of discriminating 

 horizons — that is, where in an area adjoining the standard section, ero- 

 sion at one of the epochs of unconformity has gone so far as entirely 

 to remove the immediately preceding terrane, so that the number of un- 

 conformities separating terranes is not equal in the areas compared. In 

 this case lithological characters or fossil contents must be relied upon 

 to determine whether a given bed in the syncopated section represents 

 all the time marked in the standard section by two horizons with an in- 

 termediate unconformity or whether it represents the first or second of 

 the two beds thus separated. As a consequence of this removal of one 

 formation in a series of alternating deposits and unconformities, the 

 geologist is also left in doubt as to the extent of the denudation in the 

 period of erosion preceding the deposition of the missing stratum, since 

 the subsequent erosion may have cut deeply into the subjacent terrane. 

 This is the problem presented at Clay Head, on Block island, in making 

 a correlation of certain strata with the Gay Head section. 



CLA Y HEAD SECTION. 



The geological structure of Block island is most clearly displayed at 

 Clay Head. The structure at this point is the same in kind as that made 

 out for Marthas Vineyard. The details are shown in the annexed cross- 

 section (page 210, figure 3). 



CRETACEOUS. 



The beds commonly known as Cretaceous, on account of their fossil 

 flora, are the lowest exposed on Block island. They appear at Clay Head, 

 thrown into ill shaped and overturned anticlines and synclines, the com- 

 pressed structure having a northerly dip. Nodules with impressions of 



