58 TRANSACTIONS OF THE [ Noy. 14, 
the western outcrop of the Inwood cross valley, massive and un- 
weathered, gave the following results: 
SiO,  (Fe,0,Al,0,) Cad. MgO. 
Note oid 1.34 37.23 10.26 
No.2. 3.4 0.98 39.60 10.33 
A sample of crumbling weathered sand from an adjacent out- 
crop gave 
No. 3. 9.15 3.5 27.36 8.73. 
In No. land No. 2, the percentages as CaCO, and MgCO, are: 
CaCO, MgCO, 
No. 1. 66.4 to 21.44 and 
No; 2. 70.7 to, 2169. 
The rock falls far below the MgCO, percentage ofa typical 
dolomite, and would be called a magnesian limestone. ‘he 
third analysis indicates that the lime is more soluble or suscept- 
ible to the attacks of the weather than magnesia, one-third to 
one-fourth of the CaO having gone and only about one-sixth the 
MgO, a ratio of about twotoone. Thisis corroborative of the ac- 
cepted opinion that the presence of magnesia in a mortar makes 
it more enduring. In the early part of the century quicklime 
was made from it, but it was slow in slaking and was finally 
abandoned ; but when time was allowed, it is said to have made 
a very strong and durable cement. 
The limestone contains fine crystals of white pyroxene which 
are obtained by digging in the disintegrated portion. They 
never have terminations so far as I have seen, but show excellent. 
prismatic faces. ‘The limestone also contains nodules of foetid 
quartz and felspar. 
The limestone is interbedded with the gneiss, and at 122d 
street and Lexington avenue’ to Fourth avenue, it is associated 
with a calcareous gneiss which has the structure of gneiss, but- 
effervesces. Also across the Harlem River near the Mott Haven 
Station, there is an outcrop that partakes very much of the char- 
acter of gneiss. As the sections show, the rocks are very closely 
interbedded and fade into each other. Now, as the gneiss is 
universally acknowledged to.be an altered sediment,-the conclusion 
seems unavoidable that they owe their difference simply to the 
chemical difference of the sediment, which was due to altered 
conditions of deposition, quiteanalogous to the hornblende beds, 
the granite beds, and the micaceous beds which together form 
the series. So little can be positively affirmed, however, of the 
1Mather’s Rep. 
