OSSEOUS CONGLOMERATE OF THE MIOCENE. 201 



still" the most satisfactory plane of reference in the Gay Pfead section. 

 The Miocene nowhere exceeds ten feet in thickness, and is usually, by 

 reason of erosion, much less. In certain places the Miocene is entirely 

 wanting. The Miocene ofMarthas Vineyard consists of two well charac- 

 terized lithological members, and probably a third, if this does not belong 

 to the Pliocene. The lowest of these is the " osseous conglomerate " of 

 Edward Hitchcock, to which succeeds the foraminiferal or- greensand bed. 

 The third Tertiary member may be considered as Pliocene. 



Osseous conglomerate. — This stratum, from 12 to 18 inches thick, exists 

 as a definite layer in the Gay Head section only, and there only in a 

 limited exposure, just north of the Devil's Den, between stations 33 

 and 35. Elsewhere, as in Nashaquitsa cliffs, on the south road, and 

 in the Peaked Hill district, the bed is represented in the form of rounded 

 boulders lying on the surface of the pre-Tertiary beds or in the base of 

 the greensand, where that is present. 



The inorganic detritus of the conglomerate is mainly nut-sized quartz 

 pebbles, invariably white and well rounded. Black chert and chalce- 

 donic pebbles form a noticeable element in the conglomerate, the former 

 being often fossiliferous, with corals, segments of crinoid stems, grapto- 

 lites, and sections of unidentified shells, the whole assemblage indicating, 

 in the opinion of Mr Walcott, who has examined them for me, the Silu- 

 rian age of the beds from which the pebbles were derived. 



The suggestion of a barrier to the westward of the Potomac formation 

 prior to the incursion of the Upper Cretaceous sea is helpful in account- 

 ing for these Silurian pebbles, there being as yet no evidence, as in similar 

 discoveries in the Pleistocene of New Jersey, that the pebbles may have 

 come from Silurian outcrops in the present land area of New England. 



As yet no trace of these chert pebbles has been seen in the strata un- 

 derlying the Miocene on this island. That they are remanie from some 

 earlier coast-plain formation is probable, in view of the origin of the quartz 

 pebbles in the osseous conglomerate. 



Cetacean bones give the name kt osseous " to this bed. These bones are 

 mainly vertebrae, with angular fragments of jaws, ribs, paddles, and head- 

 bones, without distinct indications of their having been buried in a 

 matrix previous to their deposition in the osseous conglomerate. The 

 porous and cellular structure of the bones would give frequent opportu- 

 nities for the adherence of a matrix distinguishable from that of the 

 osseous conglomerate had they been exposed to such materials. 



How doubtful this negative evidence is, however, must be admitted 

 when it is considered that fragments of lignite evidently derived from 

 the marine erosion of the Potomac series is of common occurrence in the 

 osseous conglomerate along with fragments of petrified wood, the latter 



