360 J. D. Dana— Geological Relations of the 
of the T shows the direction of the strike, and the stem that 
of the dip (or pitch) of the beds for the locality situated at 
their junction. Moreover, the length of the stem as compared 
with that of half the top of the T is made to give an approxi- 
mate idea of the amount of dip, according to the following 
scheme: ratio for 80°, 1:4; for 70°,1:38; for 60°, 1:2; for 
50°, 1:14; for 45°, 1:1; for 85°, 14:1; for 25°, 13:1; for 
15°, 2: 
In the following descriptions of the belts I speak : first, of the 
SOUTHERN section of the county, from New Eee Island to 
White Plains; secondly, of the MIDDLE section, from White 
Plains to Croton Lake ; and thirdly, of the NortHEeRN section, 
north of the latitude of Croton Lake. The areas are num- 
bered on the map, and these numbers are used in connection 
with the descriptions. 
The following pages contain only the general facts respect- 
ing the several belts—their position and features ; the average 
strike and dip; the characteristics of the limestone , and the 
kinds of adjoining rocks and their relations as to position. The 
details with regard to the various directions of strike and dip 
at all the points marked by symbols on the map and for other 
points not thus indicated, which make part of this paper as 
prepared, are reserved, with other details, for an Appendix. 
a. Southern Section of the County. 
Three areas or belts commence in New York Island and ex- 
tend two to four miles into Westchester County. The adjoin- 
ing rocks are mica schist and micaceous gneiss and in some 
parts thick-bedded gneiss 
Area 1.—The eastern of these three pes which may be 
called the Tremont, extends from Fordham southward to Har- 
em River, and from thence into New York ined. It reaches 
Harlem River by two lines, a western at Mott Haven and an 
eastern at the mouth of Morris Mill brook, west of Brook 
avenue. The main strike is N. 18-25° BE." 
The western of these southern terminations, or that of Mott 
Haven, shows itself just east of the railroad, south of the Mott 
Haven railroad station, in two limestone hills which will soon 
disappear from grading. The rock on the west or opposite 
side of the railroad at this ee is somewhat contorted mica 
schist; and layers of the same kind of schist are involved 
among the beds of the anhers of the hills. Near the Mott 
Haven depot, the limestone passes to the west side of the rail- 
road, and from thence it continues northward on both sides. 
The eastern line has large outcrops of purer limestone near 
The angle pri (and so elsewhere in this paper) is corrected for magnetic 
ariation. In stating the dip, only the point of the compass is mentioned in a 
general way, the exact direction being at right angles to the strike. 
