ON A HYDROMICA FROM NEW JERSEY. 
By F. W. Clarke and N. H. Darton. 
While studying the Juratrias formation in New Jersey, one of us 
(Darton) found in an old "trap" quarry at Rocky Hill a hydromica, 
which occurred under such novel conditions that it appeared to be 
worthy of investigation. It is found in veins of calcite, mainly as a 
thin coating, and adjacent to the diabase of the vein walls. The latter 
consist of more or less decomposed rock, of which the principal product 
is a soft, dark-green chloritic material. In portions of the vein the mica 
extends down the cleavage planes into the masses of calcite. A con- 
siderable amount of the calcite was thrown out during the quarrying 
operations, but only a portion of it is covered with the mica. This por- 
tion presents the appearance of having been coated with bronze paint. 
The mica occurs in minute flakes thinly matted together. Its color 
is golden bronze, although some portions are slightly greenish. The 
mineral is soft, and thinly foliated. Under the microscope it exhibits 
no definite crystalline form; and its optical properties, although not 
distinctive, suggest biotite. It appears to be biaxial, but with a very 
small axial angle, and it is pleochroic. When heated, it does not 
exfoliate. It fuses before the blowpipe, at a moderately high tempera- 
ture, to a dark-colored bead. The specific gravity was not determined. 
It is readily decomposable by hydrochloric acid. The analysis, by Mr. 
George Steiger, of material not free from calcite is subjoined. In the 
second column of figures the reduced analysis is given, titanic oxide 
and calcite being thrown out, soda recalculated to terms of potash, 
and the whole adjusted to 100 per cent. 
154 
Fou ml. 
Reduced. 
Ratios. 
Si0 2 
32.72 
.24 
8.41 
19.99 
4.24 
10.30 
5. 51 
.85 
.63 
8.21 
2.47 
6.22 
~99. 79 
40.24 
.671 
Ti0 2 
AL-0 5 
10.34 
24. 57 
' 5. 21 
.101 
. 154 
.072 
Fe 2 :5 
FeO 
CaO 
Mg-0 
6.78 
2.20 
.166 
.024 
K 2 
Na 2 
C0 2 -. 
H 2 at 100° 
3.03 
7.63 
100. 00 
.168 
.424 
H 2 above 100° 
