368 NEW YORK STATE MUSEUM 



the Potsdam in Clinton county. An examination of the shores of 

 Lake George was made for the Museum in 1S91 by Mr. E. M. 

 Blake. 



From the base of the palaeozoic upward the geologic forma- 

 tions of New York were quite accurately studied in the original 

 survey, but the work of mapping the boundaries was not very 

 carefully done, and though at the present time there is much new 

 information in the possession of those who have made special 

 studies of these formations, but little new material has been pub- 

 lished, and it has not been possible within the time at the author's 

 disposal to consult those in possession of unpublished material. 

 The author's personal observations on the palaeozoic groups 

 have been chiefly confined to the upper and lower Helderberg 

 limestones and the strata immediately adjacent to them, in Greene, 

 Albany and Schoharie counties and at various points to the west- 

 ward along the principal lines of railway. The principal guide 

 used in the preparation of the geologic base was the Agricultural 

 and Geological map of New York, published by authority of 

 the Legislature in 1814. In revising the boundaries given on this 

 map the four geologia district reports of New York have been 

 carefully studied, and from them many corrections have been 

 introduced, although the old map was based on the material con- 

 tained in those reports. This was particularly noticeable in map- 

 ping the outcrop of the upper Helderberg limestones, which on the 

 old map is shown far to the northward of Otsego and Schuyler lakes, 

 although Vanuxem reported the occurrence of the corniferous lime- 

 stone at Richfield Springs, in the town of Springfield at the head 

 of Otsego lake and at Cherry Valley ; a'similar error occurs in the 

 mapping of these limestones in Seneca county, where they are 

 shown in a straight belt between Seneca and Cayuga lakes, far 

 to the south of Seneca outlet, although Prof. James Hall reported 

 the occurrence of the upper Helderberg limestone on Seneca 

 outlet west of Waterloo. In the new map the two Helderberg 

 limestones have not been differentiated for three reasons : First, 

 because the scale of the map was too small to permit it ; second, 

 because the author had not sufficiently accurate information to 

 enable him to separate them, and, third, because the map, being 

 chiefly economic, it seemed best to represent the two limestones 



