20 NEW YOkK StATE MUSEuM 



Little opportunity to subdivide the basal gneiss member is pre- 

 sented in New York City, but through this piece of work the terms 

 " Fordham gneiss " and " Yonkers gneiss " were established. Hud- 

 son schist, Stockbridge dolomite and Lowerre quartzite were used 

 for younger members of the crystalline series. The first two terms 

 have been used consistently by all workers since, but, because of 

 the correlation difficulties introduced by those terms, the local terms 

 " Manhattan schist " and " Inwood limestone " have generally been 

 favored by students in this district in referring to the younger 

 formations. The authors of folio 83 have correlated the crystal- 

 line limestone-schist series of New York City with the Hudson River- 

 Wappinger series of the Upper Hudson valley. Tliis whole matter 

 of correlation, however, is considered by the present writers a very 

 unsettled problem and a careful summary of the arguments on both 

 sides is given in this bulletin on a later page. (See page 128.) 



The work begun in 1905 for the New York Survey on the Tarry- 

 town quadrangle was still incomplete when the exploratory investi- 

 gations of the New York City board of water supply were inaugu- 

 rated prehminary to the planning and construction of the Catskill 

 aqueduct. The senior author of this bulletin was appointed geologist 

 for this undertaking, and it thus became his duty to examine 

 critically the whole region between the Catskills and New York 

 City. Unusual opportunities were thus presented and it soon devel- 

 oped that considerable revision of the geology might be expected. 

 Exploration and construction has taken more than 12 years and, in 

 view of the fact that considerable change of conception concerning 

 the geology of the more ancient portion of the region was develop- 

 ing, it was not thought desirable to issue so permanent a publication 

 as a folio or a quadrangle bulletin until more stable hypotheses as 

 to the nature and origin and grouping of these rocks were evolved. 



In the meantime, however, several papers® of the nature of pre- 



*Berkey, Charles P. Structural and Stratigraphic Features of the Basal 

 Gneisses of the Highlands. N. Y. State Mus. Bui. 107, p. 361-78 (1907). 



Berkey, Charles P. Geology of the New York City (Catskill) Aqueduct. 

 N. Y. State Mus. Bui. 146 (1911). 



Colony, R. J. High-grade Silica Materials for Glass, Refractories, and 

 Abrasives. N. Y. State Mus. Bui. 203-204 (1917). 



Fenner, Clarence N. Tlie mode of formation of Certain Gneisses in the 

 Highlands of New Jersey. Jour. Geo!. 22:594-612. (94-202), (1914). 



Fettke, Charles R. The Manhattan Schist of Southeastern New York 

 State and Its Associated Igneous Rocks. Annals N. Y. Academy of Sciences, 

 J3: 193-260 (1913)- 



Gordtin, C. E. Geology of the Poughkeepsie Quadrangle. N. Y. State Mus. 

 Bui. 148 (1911). 



Kemp, James F. Buried Channels Beneath the Hudson and Its Tributaries. 

 Am. Jour, of Science. Ser. IV, 26:301-23 (1908). 



