86 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 
gneisses apparently continue into Lancaster County, for in looking across the Sus- 
quehanna from Slate Point a considerable thickness of light-colored rock can be 
seen overlying the slates there with a steep easterly dip." 
The structure of the slate belt proper is difficult to make out. The quartzite of the 
southeast side was not found on the northwest side during the reconnaissance. At 
the Foulk Jones & Sons' Slate Hill quarry, 2 miles northeast of Delta, there are 
some indications of anticlinal structure. Apparent bedding planesdip 30° to 40° SE. 
on the southeasl side of the quarry and similar planes curve over steeply to the north- 
wot on the northwest side, both crossing the cleavage, which is nearly vertical. 
Whether this is a minor anticline in a complex synclineor anticline or the anticlinal 
axis of the entire belt could not be determined. There is also danger of confounding 
curved joints and bedding. F razor's section along the right bank of the Susquehanna 
in York County (sheet 4, Vol. CCC) represents the slate as interbedded with chlorite 
sehiM and forming part of the northwest limb of a syncline several miles wide. 
Tin I>>/i<i and Cardiff quarries.— In June, L904, eleven concerns were operating quar- 
ries in the Peach Bottom belt. Five of these quarries or sets of quarries are located 
in Pennsylvania and six in Maryland. They lie in three parallel northeast-southwest 
lines and range from 75 t<> 120 feet across the cleavage and up to 200 feet in depth. 
The structural relations of these three hells of commercial slate are not yet apparent. 
The cleavage is uniformly vertical or dips steeply southeasl with a strike ranging 
from X. 37° to 55° E. There is usually a horizontal joint -" big flat joint" — pitching 
gently south, sometimes from 40 to 60 feel below the surface and including 2 to 3 feel ol 
crushed slate, which is evidently the result of a secondary crustal movement. Com- 
mercial slate occurs only below this joint. The "top" varies from 6 to 65 feet and 
in places includes 10 feet of slate in small fragments. Other Hat joints dip ahout 20° 
NE. or 25° SW. Longitudinal joints strike X. 48° E. and dip 00°. Diagonal ones 
strike X. 25°-30° W. and also X. 15° W.. dipping northeast at angles ranging from 
45° to 90° or 50° to 80°. Conspicuous dip joints strike about at right angles to the 
cleavage and dip 00° or southwesl ai angles of 25° or 45°. At Foulk Jones & Sons' ; 
there are vertical diagonal joints striking X. 10° E. to N. 10° W. The grain dips 
northeasl from 20° to 50°. Quartz vein.- meander in the cleavage direction. The slate 
is apt to have along the joint planes toward the surface a brownish rim, called a hem, 
from 1 to 2 inches wide and more or less parallel to the joint face. The slate breaks 
off at the inner side of the "hem," and this pari is discarded. Some of the joints ar| 
parted and contain red clay a half inch or an inch thick. The hems represent simply | 
the initial stage of weathering caused by the percolation of water from the joints, of 1 
which process the final result is the red clay. The slate of the top has generally a j 
reddish hematitic hue, and thewholebell is covered with reddish clay. The cheml 
ic;il processes involved in the passage of Peach Bottom slate into red clay as deter-] 
mined by Do. 'tor .Merrill are given on page 2.7. The effect of weathering, as shown 
under the microscope, will he given after the microscopic analyses of the slate. 
The quarrymen complain of the occurrence of what they call " black stuff," a ha 
material up to an inch thick, occurring sometimes in vertical planes diagonal to the 
cleavage. This proves to be an aggregate of much pyrite, carbonaceous matter, quart* 
chlorite, and muscovite, and is probably vein matter segregated from the adjacent 
slate. 
Peach Bottom slate. — This is very dark gray with a slightly bluish tinge. To the 
unaided eye it has a minutely granular crystalline texture, and a slightly roughish 
hut quite lustrous cleavage surface. 1 1 is markedly graphitic and contains magnetite. 
The sawn edges show a little pyrite. There is no effervescence in cold dilute hydro- 
chloric acid. It is very sonorous and does not discolor. & Under the microscope I 
a See also Frazer, as above, pp. 1.33,134. 
b The apparent contradiction oi thi La t statement with the passage of this slate into red clav h) 
weathering is explained on p. 37. 
