1889. ] NEW YORK ACADEMY OF SCIENCES. 49 
belt is usually a hard, very dark serpentine, containing very 
few mineral inclosures, and certainly in part an altered ensta- 
tite. Five or six hundred feet north of this is a much narrower 
belt of serpentine rocks containing much steatite. It is about 
two miles long. 
From the position of these rocks it seems indisputable that the 
garnetiferous schist, as well as the limestone, overlies the Pots- 
dam sandstone; though it is not uncommon to find the dips 
southerly towards the Laurentian, indicating an overturn. 
Turning now to the 8. E. side of the Laurentian axis, we find 
regular south-east dips in the Laurentian, and, adjacent to it, 
rocks very much like those on the N. W. ‘There are again two 
serpentine belts; the one nearest the axis is a hard serpentine, 
accompanied by enstatite, and evidently resulting from its alter- 
ation. Beyond this are garnetiferous schists, in which is a bed 
of serpentine and steatite, apparently an altered schist ; it is true 
that the mica-schists on the east are very extensive, those on the 
west comparatively narrow; but taken all in all, the resemblance 
is so great that I think there can be little doubt that the struc- 
ture is a simple anticlinal. 
Mr. Hall regards these serpentines and steatites as all of the 
same geological age, but if lithological evidences in adjacent lo- 
calities mean anything, this cannot be true. The hard, dark 
serpentines are exceedingly unlike the steatite of the two belts 
farther away from the Laurentian. The constant occurrence of 
this enstatite serpentine, invariably close to the Laurentian, not 
only in the region described, but also elsewhere in Pennsylvania, 
seems to be much more than a coincidence. The steatite belts, 
on the contrary, are away from the Laurentian, and in the mica 
and hornblende schists. A remarkable rock associated with the 
south-easterly steatite belt is a steatite filled with masses of black 
serpentine, sometimes having the form of staurolite. This 
occurs in immense quantity through a lineal extent of about five 
miles. The dark serpentines contain chromite; the steatites, I 
think, do not. The steatites contain a variety of minerals, the 
serpentine is almost barren. 
Between the serpentine and the Laurentian occurs a rock 
which, for want of a better name, I have termed spangled mica 
schist. It is a schist containing mica in curved crystals quite 
separate one from the other, much resembling the crystals of 
feldspar ina porphyry. At Darby Creek it is, in fact, a por- 
phyritic gneiss, but elsewhere the feldspar is in very small quan- 
tity, and minutely disseminated. 
Further eastward, and ranging to the border of the city of 
Philadelphia, is a very well defined belt of hard porphyritic 
