202 J. B. WOODWORTH — UNCONFORMITIES OF MARTHAS VINEYARD. 



showing the borings of teredo. Amber of a reddish yellow color was 

 found by Mr F. N. Balch, a student in Harvard University, in the osseous 

 conglomerate in a part of the bed where lignite fragments are of common 

 occurrence. It also was evidently derived from the lignitic beds below. 

 It remains to consider the bearing of these and a few other observations 

 upon the relations of the Miocene to the underlying pre-Tertiary strata. 



Unconformity of Miocene osseous conglomerate and underlying strata. — The 

 failure to find any trace of the Eocene in the Gay Head section is of itself 

 indication of a very widespread erosion interval between the Miocene 

 and the Cretaceous. It is a question whether the Eocene was deposited 

 in this field. There are no indubitable evidences of Eocene materials 

 carried over into the Miocene or higher beds. To the eastward, at High- 

 land light, on cape Cod, Crosby has reported the finding of drifted frag- 

 ments with a fauna considered to be of Eocene age. 



That the pre-Tertiary unconsolidated rocks were quite stripped off 

 irom southeastern Massachusetts by the beginning of the Miocene is 

 shown by the mode of occurrence of beds of this age near Marshfield, 

 where the greensand is in very close contact with the granitic and gneissic 

 floor on which the Potomac beds beneath Marthas Vineyard in all proba- 

 bility rest. It is likely that it was in this invasion of the sea that the 

 lignite fragments were introduced into the osseous conglomerate. All 

 the circumstances of contact of this bed with the underlying series and 

 its composition point to the downsinking of the land at the beginning of 

 Miocene deposition. 



Foraminiferal or greensand bed. — The foraminiferal bed is possibly of 

 deeper water origin than any sediments known at other horizons in the 

 section, for there is an almost complete absence of land waste above 

 the basal portion of the formation. The bed varies in thickness from 

 nothing to as much as ten feet. At base it is locally made up of the debris 

 of the osseous conglomerate, including rolled fragments of that stratum. 

 In its lower portion it is usually of a green color, due to the presence of 

 glauconite, but the upper half of the bed is a rusty brown, from the 

 alteration of the greensand by meteoric waters. This change evidently 

 took place before the Gay Head diastrophe, otherwise the oxidized por- 

 tion of the greensand would be now uppermost in the section where the 

 beds are inverted instead of underneath the unaltered zone where over- 

 turning has taken place. 



The casts of Macoma lyelli occur in the greensand bed in the attitude 

 of growth. The crab, Archasoplax signifera, is mainly found in the lower 

 portion of this stratum, between stations 13 and 19. 



Unconformity between Miocene greensand and osseous conglomerate. — The 

 change from the conditions controlling the deposition of the osseous 



