200 J. B. WOODWORTH UNCONFORMITIES OF MARTHAS VINEYAKD. 



like front, in which case only the surface of the advancing coarse layer 

 is in touch with water moving swiftly enough to disturb the bottom. 

 The occurrence of cross-bedding on a small scale in the coarse sands at 

 several horizons of the Gay Head section below the Tertiary makes it prob- 

 able that the marine erosion of the underlying-fresh-water beds, though 

 recognizable, was of no great depth. In the highly inclined position of 

 the strata on Marthas Vineyard differences of dip in two unconformable 

 series can hardly be detected or relied upon if occurring in any but the 

 most pronounced manner. 



The occurrence of leaves in nodules in this section is apparently due 

 to the relation brought about between the leaf-bearing clays and the 

 pyritiferous clays through the inversion of the beds. The nodules are 

 found in those portions of the section where the lignites with iron sul- 

 phides and sulphate of lime overlie by folding or overthrusting the leaf- 

 bearing beds. Thus, in the large anticline between stations 19 and 23 

 (see plate 16), nodules occur in the clays on the south of the central 

 lignite band but not in the same stratum on the north. The downward 

 percolation of the solidifying agents, derived from decomposition in the 

 lignite bed, has evidently brought about this change since the Gay Head 

 folding. This explanation is in agreement with the observation of Ward 

 that south of the New England islands the leaves do not occur in nodules 

 in regions where the Potomac beds are not overturned. 



The thickness of the plant-bearing beds in the Gay Head section must 

 be taken with due allowance for thinning and thickening in close folds 

 and for overthrusts, as well as for the probable occurrence of the marine 

 Cretaceous at the summit of the pre-Tertiary series. One hundred and 

 fifty feet is probably in excess for the beds exposed above sealevel. 



MARINE UPPER CRETACEOUS. 



The marine Upper Cretaceous described by Shaler and Foerste is char- 

 acterized by locally hardened bands of sands containing the molds of 

 fossils. Where this cementation has not taken place fossils have not 

 been observed. The beds in the Indian Hill district consist of sands 

 varying from fine to coarse textures, but with scattered larger grains of 

 quartz and abundant muscovite scales. The local development of simi- 

 lar beds above the leaf-bearing section in Gay Head cliffs makes it prob- 

 able that the marine sands are there represented. Such beds everywhere 

 appear stratigraphically beneath the Miocene beds. 



MIOCENE. 



Thickness, extent, and divisions. — The determin ation of the age of the 

 Miocene terrane by Lyell, and later with more precision by Dall, affords 



