104 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 
The grain strikes N. 35° W. At the Scotch Hill quarries (Diagram S, PL XXIV), 2 
miles north-northeast of Fair Haven, the beds strike N. 25° W., dip 5° E., cleavage 
with same strike dips 15° E. The joints are vertical, striking N. 5°-17° E. and N. 
<>7° W. At the recently opened quarries of the Brandon Slate Company, 4 miles 
south west of Brandon village, in Sudbury, the beds strike N. 55° E., dip 35° SE., 
and in places the cleavage is parallel to it. In some of the slate ledges, east-southeast 
of their first opening, there is a false cleavage striking N. 25° W. 
MINOR STRUCTURAL FEATURES. 
The purplish slates frequently have greenish ribbons — calcareous, quartzitic, and 
chloritic — an inch or two in thickness, and such ribbons run into rows or planes of 
green spots. The cause of these colored beds and spots has been discussed on page 
17. The ribboned slates here, as elsewhere, are used for flagging. 
In some of the quarries there are pyritiferous quartzite nodules a few inches in 
diameter, of lenticular form, lying in the bedding foliation and of sedimentary origin 
(see p. 33). 
Faults of no great displacement are not uncommon. These maybe reversed or 
normal (see PI. IV, C, E, and p. 29). 
Veins of milky quartz are abundant and sometimes attain large dimensions (see 
PI. V and j). 32). Their course is often very irregular. They are apt to contain 
calcite and chlorite. 
As shown by the maps, Pis. XX, XXI, the region is traversed by dikes. These 
measure from 12 to 40 and more feet in width and up to several miles in length. 
The) are of < >rdo\ ician or later date and are usually of camptonite, consisting chiefly 
of plagioclase feldspar and hornblende, with or without augite." 
These dikes are sometimes parallel to the strike or the dip joints, but more fre- 
quently to the diagonal joint-. 
Shear zones ("hogbacks") are of frequent occurrence, as is shown by the quarry 
diagrams, Pis. XXIII. XXIV. From their parallelism to the dikes and some of the 
joints they may all be due to the same earth movement (see, on shear /ones, 
PI. Y 1 1 and p. 29, and on their relation to dikes, p. 44). 
The quartzite beds, the veins, dikes, shear zones, "posts," faults, and diagonal 
joints add seriously to the difficulties presented by the overturned close folding and 
the pinching out of folds which characterize this region. False cleavage is, however, 
exceptional. 
THE WESTERN VERMONT SLATES. 
"St a-gret n " slate. — This, when freshly quarried, varies from a light gray to a slightly 
greenish gray. In some beds it i- crossed by ribbons of a dark gray or, when bedding 
and cleavage are parallel, it bears traces of organisms in dark gray. To the unaided 
eye the texture is line and the cleavage surface somewhat so, with a waxy luster. 
The sawn edges show a little pyrite. It is very slightly magnetitic, effervesces slightly 
tinder cold dilute hydrochloric acid applied to the edges, is quite sonorous and very 
tissile. After a tew years exposure it changes its color to a brownish gray, and as 
the beds discolor differently a root' covered with "sea-green" slates from different 
beds acquires a mottled color. 
Under the microscope it shows a matrix of muscovite (sericite), with brilliant 
aggregate polarization and of very fine texture, crossed here and there by obscure 
traces of bedding. This matrix incloses more or less angular grains of quartz meas- 
uring from 0.05 to 0.34 by 0.004 to 0.035 mm., but usually 0.035 by 0.013 mm., an 
occasional grain of plagioclase (lime-soda) feldspar up to 0.04 by 0.05 mm. Sec- 
tions parallel to the cleavage show many plates and rhombs of carbonate, the latter 
aSee Miss F. Bascom's study of these dike rocks in Nineteenth Ann. Kept., pt. 3, 1899, p. 223-226. 
