Clarke and Darton — Hydromica from New Jersey. 365 



Art. XL. — On a Hydromica from New Jersey; by F. W. 

 Clarke and N. H. Darton. 



While studying the Jura-trias formation in ISTew 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 cal- 

 cite. A considerable amount of the calcite was thrown out 

 during the quarrying operations, but only a portion of it is 

 covered with the mica. This portion 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 temperature, 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. 



Found. Reduced. Ratios. 



Si0 2 , 32-72 40-24 '671 



Ti0 2 -24 



A1 2 3 __. 8-41 10-34 -101 



Fe 2 O s 19-99 24-57 -154 



FeO 4-24 5-21 -072 



CaO 10-30 



MgO 5-51 6-78 -166 



K 2 -85 2-20 -024 



Na 2 0___ -63 



C0 2 8-21 .... 



H 2 at 100° 2-47 3-03 -168 



H 2 above 100° 6'22 7'63 -424 



99-79 100-00 



