204 J. B. WOOD WORTH — UNCONFORMITIES OF MARTHAS VINEYARD. 



reasons, it is perhaps probable that they are of Pliocene age. They in- 

 dicate a return to the conditions preceding the greensand. Locally there 

 are brownish clays on the same horizon, and south of the Devil's Den 

 the beds are yellowish green, apparently from the presence of dissemi- 

 nated glauconite. 



UNCONFORMITY INFERRED BETWEEN THE PLIOCENE AND MIOCENE 



GREENSAND BED. 



In the southern part of the cliffs the greensand is entirely wanting 

 and the Pliocene sands rest upon the pre-Tertiary series, with rolled 

 masses of the osseous conglomerate at their base. This unconformity 

 involving uplift after the deposition of the greensand, together with the 

 Pliocene fossils in similar non-feldspathic sands in a neighboring point 

 in the cliffs, constitutes the present evidence for the determination of 

 age here offered. 



PLEISTOCENE. 



Comparison with older beds. — The distinguishing feature of the strata 

 already described is the absence of all undecayed feldspathic sediment. 

 There are no pebbles of granite or gneiss in the Gay Head section from 

 the Potomac to the Pliocene, inclusive. Evidently during pre- Pleistocene 

 time the contributions to the sea-floor or to the basins of sedimentation 

 in this area were derived from inward portions of the coast-plain which 

 had already been deeply decayed or from decomposed portions of the 

 Paleozoic and older rocks of the pre-Potomac terrane. It is probable, 

 however, that some of the arkose beds of the Potomac derived their clays 

 in the form of grains of clastic feldspar sufficiently strong to withstand 

 mechanical transportation, their present kaolinized condition having re- 

 sulted from subsequent decay in place. Sharply contrasted with this 

 lower series of strata are the beds of Pleistocene age, throughout which 

 are abundant clastic materials of a feldspathic nature. 



The reasons for holding the lowest of these beds to be of Pleistocene 

 age are as follows : 



1. There is a lithological homology which binds these beds into a series 

 of which the morainal deposits on the surface of the island are but the 

 newest member. 



2. The derivation of the materials from the base to the glacial detritus 

 of the last ice invasion is plainly from the area of old rocks on the north 

 through a process of rapid mechanical erosion by an agent or agencies 

 capable of forming and transporting boulders, as well as thick sheets of 

 gravels and sands. 



3. The section afforded by these deposits w T ith their intercalated ero- 

 sion intervals is comparable to the record of the Pleistocene of the 



