28 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 
ever, points to a sudden rupture of large masses of rock affected in all its parts by- 
one and the same mechanical expression of energy. « 
Sedgwick termed the three commoner kinds of joints occurring in slate regions 
strike joints (joints parallel to the strike), dip joints (joints parallel to the direction of 
the dip), and diagonal joints (joints diagonal to strike and dip), terms which ought 
to be ever kept in use. <■ 
To these should be added horizontal joints ("bottom " or "flat joints") which in 
some slate regions are of much economic importance. 
Rogers called attention to the parallelism of joints to dikes in Pennsylvania, a 
parallelism previously observed in England. *' 
This is so true in western Vermont that the proximity and course of a dike can be 
foretold by the prevalence of certain diagonal joints. Joints are sometimes open and 
filled with vein matter and more frequently, when scarcely parted, have their sides 
coated with crystals of chlorite, calcite, gypsum, pyrite, barite, anatase, etc. 
Curved joints. — In exceptional instances joint planes undulate like bedding planes 
across both bedding and cleavage (see PL XVIII). This is attributed, in the Penn- 
sylvania slate region, at least, to a secondary crustal movement. The flat joint near 
the surface of the Peach Bottom slate belt, with its mass of crushed slate, is probably 
also of secondary origin. <> 
At Foulk Jones & Sons' quarry at Slate Hill, 2 miles northeast of Delta, Pa., such 
a curvature in jointing occurs as to have produced a conical structure 20 to 30 feet 
high and 15 feet in diameter at the base. In view of the peculiar curvature of frac- 
tures across the cleavage of slate resulting from the use of explosives such conical 
forms may also be the result of shock. 
Plicated joints. — In the quarry just referred to a vertical dip joint tilled with quartz 
three-twentieths of an inch thick is in plications from one-fourth to one-half inch 
wide These plications may have been due to the zigzag course of the original frac- 
ture. Some finely plicated jointing has, however, a different history. Near the 
extreme north end of the western Vermont slate belt (more exactly, one-fourth mile 
north of the northwest end of Hincum Pond, in Sudbury, Rutland County) there is 
a gray slate with a micaceous matrix in which bedding is clearly indicated by the 
alternation of small carbonaceous beds with non carbonaceous ones, and also by 
minute beds with quartz grains up to one-tenth mm. in diameter alternating with 
beds without any. All of these beds are crossed at an angle of 40° by slaty cleavage, 
and both bedding and cleavage are crossed by joints which in places are plicated, 
measuring an inch from crest to crest, but they are plicated in two directions. In 
some parts of the rock these joints are only one-fourth inch apart and are parted from 
one-tenth to one-fifth inch, the openings being filled with banded veins of quartz 
and fibrous calcite. The solution of these veins at the surface has left a series of 
plicated gaping joints crossing both bedding foliation and slaty cleavage. Under the 
microscope these joint veins are seen to consist on either side of a border of vein 
quartz, followed similarly by two bands of calcite fibers, then two bands of quartz, 
followed by a central band of gently curving calcite fibers with a central parting 
containing a little quartz at intervals. The slate between these joints also shows a 
slip cleavage crossing bedding and slaty cleavage in the general direction of the joints, 
but not plicated. The history of this slate thus appears to be: (1 ) The folding of a 
mass of marine sediments of alternating composition and the formation of slaty 
cleavage at an angle of 40° to the folded beds; (2) compression along the strike 
aLoretz: Uber Schieferung, pp. 98-100. 
bA Synopsis of Classification of the British Paleozoic Rocks, by Adam Sedgwick, London. 1885, 
p. 35. 
cH. D. Rogers, Geology of Pennsylvania, vol. 2, pt. 2, p. 912, fig. Vis, joints in red shale parallel to 
dike; also fig. 719, joints 'in argillaceous sandstone; 1858. 
d See on curved joint planes in the Peach Bottom district: Edward B. Mathews, Maryland Geol. 
Survey, vol. 2, 1898, p. 223, PI. XXii, fig. 1. 
