NATURAL SCIENCES OP PHILADELPHIA. 303 



faces. The steatite in which it was imbedded, and the serpentine 

 Itself, contained ferruginous dolomite or breunnerite. 



On the northeast side of Mill Creek, a portion of the rock in 

 place was found very much Aveathered on the surface, the steatite 

 being cavernous and decomposed, and very soft and brittle, 

 owing, probably, to a large admixture of ferruginous dolomite, 

 but the serpentine gone entirely, save a little pulverulent oxide 

 of iron ; the cavities were nearly all lenticular in shape, but too 

 regular to be other than matrices of cr^'stals, while in two cases 

 distinct cruciform cavities with anoles of about 60° were ob- 

 served. The portions of rock containing these were cut out, and 

 in one of them lead was poured, and a cast obtained, which, while 

 irregular and rough, was a fac-simile in metal of the common 

 cruciform twins of staurolite. Portions of the same rock which 

 had not altered were found containing the serpentine in distinct 

 crystals, irregular in outline, but twinned at angles of about 60°- 



Serjjenfine. — About a half mile above the soapstone quarries 

 on the Schuylkill, occurs a ridge of serpentine which I believe 

 has never been described. Its first appearance is at a slight cut- 

 ting of the Philadelphia and Reading Railroad, opposite and a little 

 above Lafayette station on the Norristown Railroad, and just 

 below the vein of granite which crosses the Schuylkill below 

 Spring Mill, and crops out at several points on the elevated hill 

 of gneiss, which crosses the Schuylkill at Spring Mills. About 

 a hundred yards S.W. from the river, it has been cut through by 

 a small stream, and here it has been quarried as a building-stone, 

 and is well exposed. The serpentine is very compact, at times 

 slaty, of a very dark green, almost black, color, unlike that of any 

 other ridge of the neighborhood, and resembling that of the Hartz. 

 Intermixed, and also at the bottom of the quarry on the north- 

 west, is a foliated mineral resembling Schiller spar, or serpen- 

 tine, pseudomorphous after Pyroxene or Hornblende. The ser- 

 pentine dips steeply toward the southeast, and at this point rises 

 probably one hundred and fifty feet above the valley, abruptly 

 and precipitously on the N.W., sloping on the S.E., where talcose 

 and micaceous schists rest against it. About a half mile from 

 the river, the ridge widens, the slopes are more gentle, and, for a 

 short distance, the serpentine is hidden; but it again crops out 

 about one mile from the river, where a road, parallel to the river, 

 crosses the stream which has been mentioned, and which skirts 

 1872.] 



