56 NEW YORK STATE MUSEUM 



characters as in the Helderbergs and much farther west and south 

 is undeniable evidence that the Helderberg sea extended farther 

 east than the present Rensselaer plateau and with unchanged or but 

 little changed conditions. In this connection it is further to be con- 

 sidered that the Rensselaer grit plateau has clearly Its main exten- 

 sion in a north and south direction, (according to Dale, there is a 

 further outlier in Vermont) and represents a deposit in a long sub- 

 meridional Appalachian trough. Its pebbles . of coarse and fine 

 gneiss came from a short distance and the numerous Lower Cambric 

 pebbles probably from places north of the plateau. It is therefore 

 the deposit of an embayment which may have received its materials 

 from the north. The entire absence of fossils in the nearby Becraft 

 mountain formations is a further argument against correlation with 

 the latter, as it indicates estuarine conditions greatly different from 

 the marine conditions of the Helderbergian sea depositing the 

 Becraft mountain rocks. 



These indications of estuarine conditions in the Rensselaer basin, 

 consisting in the alternation of red and other highly colored shales 

 with coarse grits and conglomerate and the barrenness of the beds 

 in fossils, suggest a possible identity with the Catskill beds which 

 loom up across the Hudson, thousands of feet thick and only 30 

 miles away from the outlier of Austerlitz. There is no doubt that 

 the deposits of the Catskill estuary must have extended to and be- 

 yond the Rensselaer grit plateau ; the main extension of that estuary 

 a^ already shown by the writer was in the same direction as the 

 R.ensselaer plateau. The thickness of the Rensselaer grit corre- 

 sponds better with that of the Catskill beds than with any other, 

 the lithologic characters are similar, and both have in common the 

 barrenness in fossils. 



From the location of the Rensselaer grit plateau relative to the 

 Siluric and Devonic rocks of New York and its lithologic char- 

 acters, the Rensselaer grit would be most naturally connected with 

 one or another of the two Devonic phenomena referred to, viz, the 

 Oriskany invasion or the Catskill embayment. 



Eurypterus fauna of the Shawangunk grit. Reference has 

 been made above to the discovery, after the determination of the 

 age of the Shawangunk grit of Orange county as probably equiva- 

 lent to the Salina of central New York, of beds bearing Eurypterus 

 intercalated in these grits. In another place I have given a full 

 account of the stratigraphy and character of this fauna and here 

 briefly summarize the principal facts of this very noteworthy and 



