80 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 
verse to the cleavage and parallel to the grain; also chlorite scales transverse both 
to grain and cleavage, with inclusions of needle-like crystals, probably rutile, cross- 
ing one another, usually at 60° or 120°. All these chlorite scales average about 18 
per square millimeter, and measure up to 0.17 by 0.1 mm. The pyrite spherules 
measure up to 0.01 mm., and number about 150 per square millimeter. Rutile needles 
also occur. 
These constituents, arranged in descending order of abundance, appear to be mus- 
covite (sericite), carbonate, quartz, chlorite, pyrite, rutile, and magnetite. 
The product of the Albion quarry, at least that of the western end, is -used for 
rooting. 
Weathering at Pen Argyl. — Some of the other quarries at Pen Argyl show the passage 
of the black slate by weathering into a yellowish light brown, and even white shaly 
substance. The dumps afford specimens showing all gradations from black slate to 
white rock. The weathered zone in these quarries measures up to 40 feet in thick- 
ness. At one of them the beds of the truncated southern limb of an overturned 
syncline dip about 55° south, while the cleavage dips leg's than 10° in the same 
direction. The weathered top measures between 30 and 40 feet, and is covered with 
from 5 to 10 feet of subsoil. The weathering acids, taking advantage of the position 
of the bedding, have followed one of the ribbons and penetrated the workable slates 
to a depth of 60 feet below the weathered zone, and have also commenced to work 
horizontally along the cleavage. This has resulted in the formation along the ribbon 
of a central bed of reddish-yellow ocher about 1 foot thick, with a 6-inch band of 
white on either side of it, The white portions here probably represent less ferru 
ginous parts of the original sediment, and not a later stage of weathering, for similar 
alternations of yellowish and white belts of weathered material occur in hand 
specimens crossing the cleavage. The workable black slate is thus traversed here by 
a 2-foot bed of yellow and white ochraceous material at an angle of 65° to the cleav 
age as the effect of weathering upon material and structure. The process here ii 
mainly decarbonization. (See further, on the subject of weathering, p. 37.) 
In this connection it should be stated that in the "soft vein " quarries generally, 
judging from the dumps, the amount of iron and lime in the ribbon varies greatly. J 
In some cases pyrite or siderite predominates and limonitic staining ensues, in others 
lime and a white calcareous crust forms. The slate flags on the sidewalks in the 
village of Bangor wear along the ribbons more rapidly than along the slaty portions. 
Chapman. — This is in the "hard vein" belt. The principal quarry is operated by 
the Chapman Slate Company. It measures from 700 to 800 feet along a longitudinal 
joint striking N. 63° E. across it, and is about 200 feet wide and 300 feet deep. In 
the center of the quarry is a completely overturned close fold, syncline, or anti- 
cline, still visible on the south-southeast wall of the quarry, striking N. 75°-78° EJ 
pitching gently eastward; but the lower limb of this- fold opens out on the east- 
northeast and west-southwest walls so as to dip steeply northwest. This fold is 
crossed by cleavage striking N. 65° E. and dipping 14°-15° SSE. The principal I 
joints are longitudinal, striking N. 63° E. and dipping 00°, forming two of the sides - 
of the quarry. The grain strikes N. 37°-53° W. and dips 90°. The largest bed, 
which, although containing very small ribbons, can be used for rooting slate, meas- 
ures 16 feet. The number of ribbons exposed on the east-northeast wall, where the 
fold opens out, is 112, averaging a little over 2 per foot of slate." 
Some quartz veins lying in the cleavage show a columnar structure parallel to the j 
side of the vein. The "top" in an adjacent quarry of the same company measures < 
fully 50 feet. 
The slate is very dark gray, but is crossed by frequent ribbons a trifle darker and j 
measuring from one-eighth to one-half inch, which, however, scarcely deflect the 
"Figures through the courtesy of Mr. Richard Chapman, superintendent. 
