J. J. Stevenson — Use of the Name u Catskill." 335 



The constancy of the conditions within the Catskill Mountain 

 area from the very beginning of the Upper Devonian made 

 the relations of its rocks to those of other regions very per- 

 plexing. As Prof. Hall has shown, even before the close of 

 the Hamilton, the conditions became unfavorable to animal 

 and vegetable life and favorable to the formation of red beds ; 

 and these conditions, with rare interruptions, continued until 

 the end of the Devonian. But as one recedes from the area 

 of the Catskills, he finds that these conditions did not begin 

 alike early everywhere* and that the horizon at which red 

 rocks become a marked feature, varies stratigraphically not 

 less than 3000 feet ; and the upper limit of animal life varies 

 even to nearly 4000 feet, there being localities where some 

 mol lu scan forms were able to sustain themselves amid the red 

 beds at the very top of the section. 



But this was no hap-hazard variation. The writer has shown 

 that the conditions beginning so early in the Catskills of New 

 York, spread toward the south and west slowly but steadily 

 until at the close of the Devonian, they prevailed along the 

 easterly outcrop to beyond the New River in Virginia and 

 westward to the limits already given as those of the Catskill. 

 The characteristic fossil of the Oneonta sandstone (Portage) in 

 New York is the Amphigenia, commonly thought to be a 

 freshwater form. As the conditions favorable to the existence 

 of this mollusc extended, the geographical distribution must 

 have become greater, so that there was no reason for surprise 

 when, in 1881, the writer discovered that form near the sum- 

 mit of the Montrose sandstone in southwestern Pennsylvania, 

 several thousand feet above the Portage, its horizon in the 

 Catskills. 



But these conditions spread very slowly ; Mr. Darton's sec- 

 tion from Broome County eastward in New York exhibits the 

 changes observed previously in northern Pennsylvania by I. C. 

 White and in southern Pennsylvania by Stevenson. So slowly 

 did they spread, that for a very long period, lasting almost to 

 the end of the Chemung, as limited above by Yanuxem, they 

 had reached southwestward to barely 75 miles within Penn- 

 sylvania and westward but little further — an utterly insignifi- 

 cant area when we remember that the whole series has been 

 examined along the easterly outcrop for 500 miles further 

 southwestwardly and that, by means of the oil-borings and 

 the successive anticlinals, the section is familiar to the south- 

 western limit of Pennsylvania ; while the tracing along the 

 northern line of Pennsylvania and the southern line of New 

 York is sufficiently simple. Southwardly in Pennsylvania, 

 one comes quickly to sections showing fossiliferous beds in the 



* Stevenson, loc. cit. 



