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NIW TOEK STATE MTSEUM 



of geologic detail is far from complete. This is especially true 

 of the Pre-cambrian formations which consist of metamorphic 

 and igneous rocks. This is not, however, very remarkable when 

 we consider that accurate methods of rock study and classifica- 

 tion have had their greatest development sirce 1873, when 

 through the labors of Zirkel and Rosenbusch the microscope was 

 successfully applied to the study of rocks. In mapping the Pre- 

 cambrian formations of New York the author is, therefore, un- 

 able to give any great amount of detail. In Westchester, 

 Putnam and southern Dutchess counties his personal studies dur- 

 ing a number of years, with the assistance of Messrs. E. M. Blake 

 and H. Ries, have enabled him to differentiate the areas of meta- 

 morphosed palasozoic limestones and schists from the subjacent 

 gneisses which can be traced northward through Westchester 

 county and are apparently continuous with the banded gneisses 

 which rest upon the granite of Putnam county. The 

 small scale of the map makes it impossible to show the full 

 detail of these narrow belts of rock which owe their existence 

 to the folding and erosion which has taken place within 

 that region. Within the Pre-cambrian area of Putnam county, 

 which is generally known as the " Highlands," in addition to the 

 banded gneisses which contain the beds of magnetite, there are 

 large masses of granite which appear along the axes of the moun- 

 tain folds, being flanked by the gneisses. The author 

 regards these as igneous granites made plastic in the process 

 of mountain making which created the folds in which they occur. 

 No attempt has been made to differentiate these granites in the 

 mapping, nor has any field work been undertaken with this end 

 in view. The southwestern extension of this Pre-cambrian area 

 through Rockland and Orange counties into Xew Jersey has pre- 

 cisely the same component rocks and structure. Besides the 

 " Highlands " Pre-cambrian area just mentioned, there is the 

 greater area of the Adirondack wilderness. This is known to 

 include two principal formations of Pre-cambrian age. First, an 

 area of metamorphic rocks, extending from Lake Champlain 

 to the Black river and from southern Fulton county nearly 

 to the Canadian boundary. Secondly, in the eastern part of the 

 wilderness and touching at two points the shore of Lake Cham- 

 plain is a mass of basic plutonic rock chiefly composed of hyper- 



