96 TiiE MiDDLK Devonian Deposits of Maryland 



lachian province, species of Onondaga type invading and mingling witli 

 those of Marcellus type in tlie latter sediments. Upon the termination 

 of the Onondaga, Marcellus species persisted in the east and the later shale 

 deposits of New York and Maryland were formed. The difference be- 

 tween the faunas of tlie shales of New York and Maryland is to be ex- 

 plained by the ditferences of habitat, black muds prevailing in New York 

 while more arenaceous clays were deposited in Maryland. The Maryland 

 beds may also be older in part than the corresponding shale deposits of 

 New York. This explanation would fully harmonize with the conditions 

 shown by Clarke to exist in New York and would explain the undoubted 

 fact that the sediments were laid down in Mar}dand in a basin occupied 

 simultaneously by numerous species of both the Onondaga and ]\larcellus 

 faunas. 



Agoniatitcs expnn-siis appears in largest numbers in the upper 50 feet 

 of this member where it is associated with Bactntes aciculatus precisely 

 as it occurs in the Goniatite limestone of New York, to which horizon the 

 former species is essentially restricted in that State. The limestone is 

 impure and argillaceous and also increases in thickness in the western 

 sections as in New York, being best seen in Maryland at 21st Bridge. 

 Although it is not possible to affirm with confidence that the Goniatite 

 limestone is the same in Maryland and New Y''ork, nevertheless their 

 resemblance is very suggestive and it is believed that they may represent 

 the same horizon. If this is true then the upper beds represent the hori- 

 zon of the top of the Onondaga of western New York and of the Goniatite 

 limestone in the Marcellus of central New York, while the lower beds may 

 be older than the base of the Marcellus of that State. It also seems pos- 

 sible that a hiatus may exist in the Mar\dand section as is suggested by 

 the apparent unconformity at the top of the Oriskany. The suggested 

 relations of the New York and Maryland sections are shown in the diagram 

 on the following page.^ It must be remembered, however, that the precise 

 correlation is but tentative and is not regarded as proved. 



' The figure illustrating the conditions in New York is taken from Clarke's 

 discussion in Bull. N. Y. State Mus., No. 49, 1901, p. 137. 



