NON-MARINE AND MARINE CRETACEOUS BEDS. 199 



conformities, some of which were in doubt in 1889. These will be re- 

 viewed in the relation which they bear to underlying and overlying 

 formations. 



NON-MARINE PLANT-BEARING BEDS. 



The lowest beds exposed above sealevel on Marthas Vineyard, so far 

 as observation has yet determined, are lignitic and leaf-bearing clays, 

 the island series of Professor Ward, held to be Lower Cretaceous on ac- 

 count of their fossil flora. In the legend accompanying the section of 

 1889 these beds were placed in the Cretaceous because they were recog- 

 nized as underlying the marine Upper Cretaceous on Marthas Vineyard. 

 At the same time David White published a description of the plants 

 found in these beds and referred them to the Lower Cretaceous. They 

 are the beds now referred by Marsh to the Jurassic on the ground that 

 they are the stratigraphic equivalents of the Potomac in Maryland, in 

 certain strata of which he finds bones of Dinosaurs of Jurassic type. 



With the conclusions of the paleontologists, the geologist in this field 

 can have no difference of opinion. The lignitic and leaf-bearing clays 

 of Marthas Vineyard underlie the marine Upper Cretaceous. The beds 

 are obviously newer than the Newark group of' southern New England. 

 It is simply a question whether the fossil contents of the Potomac group 

 afford a means of close correlation with accepted Cretaceous and Jurassic 

 sections. There are a few observations upon the relations of the Creta- 

 ceous beds at Gay Head which are pertinent to this problem. 



UNCONFORMITY INFERRED BETWEEN NON-MARINE BEDS AND MARINE 



CRETACEOUS. 



The plane of demarcation between the non-marine plant-bearing beds 

 and the overlying marine sands and clays of Upper Cretaceous age on 

 Marthas Vineyard has not been definitely traced. That such a surface 

 exists with some signs of local unconformity, at least, is to be inferred 

 from the existence of water-worn patches of the lignitic materials in the 

 sands at Gay Head, which, though without marine fossils, are litholog- 

 ically identical with the fossiliferous strata in Indian hill. Again, at 

 Indian hill traces of the lignite are found in the matrix enclosing the 

 marine fauna, showing that the plant-bearing series had been attacked 

 in the invasion of their area by the sea. In general, where coarse, grav- 

 elly sands like those of the Upper Cretaceous succeed unconsolidated 

 clays, it is to be expected that the strong currents which were able to 

 bring coarse sediments to a given position upon the sea-floor were strong 

 enough to remove the finer particles already laid down by weaker cur- 

 rents. An exception to this action arises where the coarse sediments 

 invade a clay bottom in the form of a layer with a cross-bedded delta- 



