HUDSON RIVER BEDS NEAR ALBANY 565' 



the writer trusts, have been secured in suflScient quantity to^ 

 justify a reopening of the discussion. 



Both investigators based their defense of the term on 

 the supposition that the entire series of the Hudson 

 river shales, though in diminished thickness, continued 

 into the Mohawk valley, and that these, like the Mohawk 

 valley shales, represented only the lapse of time be- 

 tween the Trenton audi Medina formations. This supposition, 

 though at that time warranted by the facts at hand, has now 

 proved to be only partly correct; for only the Utica and Lorraine 

 shales of the Hudson valley continue westward, while the 

 apparently enormous mass of Trenton shales does not leave its 

 confines. The term could then be applied only to the shales in 

 the Hudson valley north of Cohoes and in the hills on the west 

 side of the valley as far south as Albany, while the shales all 



These two differential features of the Utica faunas of the Hudson valley, 

 viz, the ascension of the Trenton forms and the restriction of the different 

 faunal composition to the marginal region, are evidently to be traced to the 

 same cause. The assumption that these Trenton forms continued to live 

 in Utica time in the adjoining Atlantic basin, while they had become 

 extinct on the American continental platform, and thus were enabled to 

 leave their shells in the deposits of the eastern continental shelf, seems to 

 offer a reasonable explanation. 



At the same time it is evident that in this region the change in physical 

 conditions from the Trenton to the Utica epoch was by no means so pro- 

 found as in the Mohawk valley and west of it; for there Utica mud shales 

 follow more or less pure Trenton limestones, and here the deposition of 

 clastic sediments was uninterrupted from the lower Trenton to the top of 

 the Lorraine. Such forms as in Trenton time were accustomed to live under 

 conditions that led to the deposition of mud and sand had of course a 

 much greater chance to continue living when the Utica time was ushered 

 in, in such easy stages as are apparent at Mechanicsville and on Green 

 Island, than the faunas farther west. But it is, then, pertinent to ask 

 why these forms did not wander with their new Utican companions, which 

 came across these marginal areas on the continental platform, into the 

 interior. This latter fact and the restriction of the graptolite, C o r y n - 

 oides curtus, to the east suggest again that this regional difference 

 is of a character similar to that between the Hudson river Trenton and 

 the continental Trenton, viz, a difference between the oceanic fauna, 

 encroaching on the continental margin, and the fauna of a shallow con- 

 tinental and partially inclosed sea. 



