PENNSYLVANIA. 
87 
shows a matrix of muscovite (sericite), of rather coarse almost schistose texture, but 
with brilliant aggregate polarization. There is much graphitic material in fine par- 
ticles and also much quartz, some of which is probably secondary, in grains measur- 
ing up to 0.05 and sometimes 0.12 by 0.03 mm. Chlorite can in some specimens 
be made out mingled with the muscovite. There are tabular crystals of magnetite 
measuring up to 0.11 by 0.02 mm., and also lenses of magnetite of larger size, but 
the most conspicuous feature is the abundance of crystals of andalusite (AL0 3 , Si0 2 , 
silicate of alumina) measuring from 0.008 by 0.001 mm. and under, up to 0.11 by 
0.03 mm. and exceptionally 0.2 by 0.02 mm., and numbering from 15 to 80 per square 
mm. These crystals contain opaque inclusions arranged diagonally to their vertical 
axes, and are often in lenses, measuring from 0.04 to 0.25 mm. long by 0.04 to 0.1 nun. 
wide, which consist variously of chlorite interleaved with muscovite with some quartz 
at the ends, or mainly of quartz, the andalusite crystal having a plate of muscovite 
parallel to it on one or both sides; or the lens consists of muscovite, the crystal being 
bordered by quartz, or, finally, entirely of secondary muscovite. When not in lenses 
the andalusites are apt to have a plate of muscovite on one or both sides. These crys- 
ig. 10.— Transverse section of Peach Bottom rooting slate, showing andalusite crystals in a rather 
coarse micaceous matrix. Enlargement about 113 diameters. Some of these crystals are in lenses 
of quartz and muscovite and several show inclusions. 
lis lie with their long axes across the cleavage at various angles and with or across 
:ie grain, but the axes of the lenses which inclose them are usually parallel to the 
leavage (see fig. 10). Pyrite is difficult to distinguish under incident light from the 
ndalusite when this is rendered opaque by inclusions. Kutile occurs in irregular 
rystalline masses up to 0.09 by 0.04 mm., hut no needles were detected. There is 
irely a rounded grain of zircon; no carbonate detected. 
The important constituents of this slate, arranged in descending order of abun- 
ince, appear to be muscovite (sericite), quartz, graphite, andalusite. magnetite, 
yrite, with accessory rutile and zircon. Fig. 10 will serve to show the andalusite 
ystals which are so characteristic of this slate. The highly crystalline character of 
te matrix and the absence of carbonate indicate a very durable slate. 
The partially weathered slate of the "top" and "hems" shows, under the micro- 
ope, the magnetite crystals and lenses passing into hematite and the andalusite 
jcoming limonitic from the oxidation of its inclusions, which must, therefore, he 
rruginous and the matrix stained with hematite and limonite from the above 
terations. 
