MIOCENE GREENSAND AND OSSEOUS CONGLOMERATE. 203 



conglomerate to those which permitted the accumulation of several feet 

 of foraminiferal casts almost without mechanical waste from the land 

 involves the exclusion of ordinary shore processes by a depression of 

 the land moving the shoreline inward ; by a depression of the sea-bottom 

 to a depth beyond the reach of land sediments, or by the production of 

 an offshore barrier, with a warm quiet water lagoon in which organic 

 growth went on without admixture of sediments from the erosion of the 

 sides of the channel. This last case is represented by the condition of 

 things which exists in the straits between the Queensland shore and the 

 Australian Great Barrier Reef. Here, with a lagoon from 20 to 30 miles 

 wide on the north and from 50 to 70 on the south, with a depth of water 

 varying from 10 to 25 fathoms on the north and increasing southward 

 to as much as 40 and 60 fathoms, Jukes dredged on the sandy bottom 

 " bagful after bagful of orbitoloides, which also often form the sand on 

 the mainland and the islands." 



There is a tendency to assume that the presence of foraminiferal beds 

 indicates tolerably deep water, but I am informed by Mr Alexander 

 Agassiz that this is by no means a necessary conclusion, since great 

 quantities of these forms may be driven towards shore and there de- 

 stroyed so as to form a stratum. The occurrence of disrupted fragments 

 of the osseous conglomerate in the base of the greensand bed on Gay 

 Head is presumptive evidence that there was a shallowing rather than 

 a deepening of the water in the change from the conditions of the earlier 

 to the later Miocence deposits in this section. That this change was 

 more than local is shown by the same distribution of rolled fragments 

 of the osseous conglomerate in the greensand beds in the southern part 

 of Marthas Vineyard. 



Professor William B. Clark has suggested to me, from the case of the 

 New Jersey Miocene, that the Miocene greensand bed on Marthas Vine- 

 yard may owe its origin in part to the redeposition of Cretaceous green- 

 sand beds now wanting on the Massachusetts coast. In the absence of 

 specific identifications of the forms in the Gay Head section, it is im- 

 possible at present to discuss this question. Mr F. J. H. Merrill has 

 also advocated the secondary nature of the greensand bed.* 



PLIOCENE. 



The existence of Pliocene strata in the Gay Head section rests upon 

 Dall's identification of fossils found in sands lying upon the greensand 

 bed. Similar sands without known fossils occur to the southward in 

 the cliffs between the Pleistocene and the identified Miocene. It is 

 doubtful whether these sands are Miocene or Pliocene. For structural 



* This Bulletin, vol. i, 1888, p. 556. 



