CURRENT PRE-CAMBRIAN LITERATURE 529 
varieties with increasing amounts of bisilicates to basic olivine gabbros. 
The gabbros vary from massive to gneissoid rocks which are difficult 
to discriminate from some of the gneisses of series 1. These rocks 
contain the titaniferous iron ores. ‘They are intrusive in series 1 and 
2. Resting unconformably upon 1, 2, and 3 is the Potsdam sandstone. 
Kemp‘ describes the geology of the magnetites near Port Henry, 
N. Y., and especially those of Mineville in the Adirondacks of New 
York. The oldest rocks present in the district are quartzose gneisses 
and white crystalline limestones, with perhaps some more _ basic 
gneisses. The limestones appear to lie largely in the upper part of 
this group, but some of them are certainly below the other members. 
The acidic gneisses may have been granites or quartz-diorites. The 
gneiss and limestone group is cut by anorthosite intrusives, and both 
are in turn cut by gabbro intrusives. Trap dikes, usually of small 
width, are very common in this district. The age of these dikes is 
undetermined, but it seems probable that they may be of two ages, 
pre-Potsdam and post-Utica. Overlying unconformably all of the 
above described rocks is the Potsdam sandstone. 
Darton? describes and maps the faulted region of Herkimer, Ful- 
ton, Montgomery, and Saratoga counties, New York. Laurentian 
rocks occupy the northern part of the area, forming the floor for a 
succession of sandstones, limestones, and shales, which dip to the 
south at a very moderate angle. 
Kemp? describes the East River and Blackwell’s Island section, 
made by an underground tunnel at 7oth street, New York City. 
Under the west channel is a fine grained mica-gneiss, containing 
pegmatite seams. Under Blackwell’s Island and the adjacent waters 
is a gray gneiss. In the center of the east channel is a dolomite, 
*The geology of the magnetites near Port Henry, N. Y., and especially those of 
Mineville, by J. F. Kemp: Trans. Am. Inst. Min. Engineers, Chicago meeting, Feb. 
1897, p. 58. 
2A preliminary description of the faulted region of Herkimer, Fulton, Mont- 
gomery, and Saratoga counties, by N. H. Darton: 14th Ann. Rept. Geol. Survey of 
New York, for 1894, pp. 31-56, 1896. With geological map. Published in the 48th 
Ann. Rept. N. Y. State Museum, 1895. 
3The geological section of the East River at 7oth Street, New York, by J. F. 
Kemp: ‘Trans. N. Y. Acad. Sci., Vol. 14, 1895, pp. 273-276. 
