12 NEW YOUK STATE MUSEUM 



Pre-Cambrian and metaraorphic rocks of southeastern New York 



Mather, in his final report of 1842, described the rocks of this 

 area under two heads: nietamorphic and primary. In the former 

 he included the mica slates, quartzites and crystalline lime- 

 stones; in the latter the granites, gneisses and igneous rocks. 

 Though in deference to one of his colleagues, he had given spaci' 

 to the Taconic system, he stated that he believed it to consist 

 merely of altered representatives of Champlain (Cambro-Silu- 

 rian) age, and further says that the " metamorphic " rocks are 

 probably of the same age, but still more highly altered. On 

 the 1842 map, however, only the limestones are separated out 

 and colored distinct from the " Primary gneisses and granites.'' 



Forty years of reactionary ideas elapsed before the work of 

 Dana and the writer verified Mather's statements, the interval 

 being filled in with the publication of schemes of classification 

 and theories of origin, in which the names Laurentian, Huronian, 

 Norian and Montalban figured prominently. The difficulty of 

 establishing these theories without actual field work was appar- 

 ently not manifest to some of the authors. 



The history of modern geologic work in the region dates back 

 less than a quarter century. Dana, in the extension of his- 

 work on the Taconic rocks, carefully worked over Dutchess, 

 Putnam, Westchester and New York counties; and correlated 

 the crystalline limestones of the last two with the Cambro- 

 Silurian limestones of Dutchess county. Failing, however, to 

 differentiate the mica schist (Manhattan or Hudson) overlying 

 the limestones (Inwood or Stockbridge) from the gneisses (Ford- 

 ham) underlying them, his work lacked completeness. This 

 want was supplied by the writer, who recognized the dissimi- 

 larity and true stratigraphic positions of the two noncalcareous- 

 formations, and correlated the Manhattan schist with the Hud- 

 son river slates and shales.*^ Detecting also the presence ia 

 these counties of a comparatively thin bed of quartzite (Lowerre) 

 immediately below the limestones, he inferred its equivalence 

 to the Cambrian quartzite of Dutchess county. The series was 



^\m. Jour. Scl. Ser. 3. 39:380; N. Y. State Mus. 50th An. Rep't. 1:21-31. 



