52 NEW YORK STATE MUSEUM 



festation of igneous action in the Precambric era came intrusions 

 of diabase, reaching the surface no doubt and forming lavas, but 

 now found only in the filled-up channels or dikes below the old 

 outlets. There are countless numbers of these dikes in the eastern 

 and northern Adirondacks. They are all younger than the last 

 period of general metamorphism and have remained practically 

 unchanged, except by surface weathering. 



The Highlands region, according to the more recent investiga- 

 tions which have been carried on chiefly by Berkey,^ presents quite 

 an array of Precambric rocks quite similar to those already enu- 

 merated for the Adirondacks, except that here the acid or more 

 siliceous types greatly predominate in the igneous complex. The 

 main element in the geology of the central area is a group of 

 gneisses, which are known to be composite, though they have not 

 been definitely classified. They include the oldest formations and 

 such contrasting representatives as the massive granite gneiss of 

 Storm King in the northern section and the foliated banded Ford- 

 ham gneiss which has sedimentary affinities and is widely distributed 

 in Westchester county. There is also a considerable development 

 of mixed types, probably an involved aggregate of igneous and 

 sedimentary derivatives. Small bands of crystalline limestone and 

 quartzite are found in the central Highlands and, with the older 

 sedimentary gneisses, constitute a series which is placed by Berkey 

 in the Grenville. There seems to be no recognizable parts of the 

 Arohean in this section. The Precambric intrusives are mostly 

 granites, with a few syenites and diorites. Igneous activities did 

 not cease, however, with the close of the Precambric, as was the 

 case in the Adirondacks, but continued as late at least as Siluric 

 times. 



The older gneisses in the region are succeeded by a group of 

 metamorphosed sedimentary formations including crystalline lime- 

 stones, schists and quartzites. These find strong representation in 

 the southern section where the limestones have some importance 

 for building marbles and lime-burning. While they are certainly 

 younger than the gneisses of the central Highlands, their precise 

 place and relations are not altogether clear. It is possible, as has 

 been suggested by Merrill, that they are the more thoroughly 

 metamorphosed equivalents of the Hudson river beds to the north 

 of the Highlands, in which case they belong to the Cambric and 



1 See specially " Structure and Stratigraphic Features of the Basal Gneisses 

 of the Highlands." N. Y. State Mus. Bui. 107/ 1907. 



