STRUCTURE. 19 
of course, assumes that the slate beds still retain the position they had when slaty 
cleavage was set up in them. 
PI. II, B, shows a ledge of green roofing slate with a cleavage' flipping 20° east 
(strike N. 15° W. ); joints striking N. 30° E. and dipping 45° east. The upper part 
is a quartz sandstone or grit, with calcareous concretions containing Lower Cambrian 
trilobites. The direction of the axial planes of the calcereous bodies and the direc- 
tion of the line of contact between the slate and sandstone show the direction of the 
bedding to be horizontal. 
The "ribbons" of the Pennsylvania quarries shown in figs. 6-9 and Pis. XIII, XIV, 
XVIII-XX, consist of small beds of quartz sandstone or grit in which the particles 
are held together by a calcareous and sometimes a sericitic cement, the original clay 
having gone into muscovite. Sometimes the slate beds are separated, as in the Ver- 
mont and New York quarries (Pis. XXIII, XXIV), by beds of calcareous quartzite 
reaching several inches in thickness and consisting of grains of quartz sand cemented 
together by secondary quartz and calcite and other carbonates. Such beds may be 
but a fraction of an inch in thickness, and consist mainly of carbonate, as in the 
syncline at West Pawlet, Vt., shown in PI. XXV. In some cases bedding is indi- 
cated simply by a variation in the amount of lime in successive beds, as in the 
syncline at West Castleton, Vt., shown in PI. III. Here the solution of the lime by 
the acids of the atmosphere has etched the joint face, as it were, and the more 
calcareous beds thus stand back from the less calcareous ones. 
Planes of bedding may be indicated by the position of fossils, as the brachiopods 
and trilobites at the Arvonia quarries in Virginia, or by bifurcating impressions, 
possibly made by seaweeds, and serpentine ones by annelids, or by the small black 
beds in green slate due to the decomposition of various marine organisms, all of which 
may be seen in the slate quarries of western Vermont and eastern New York. 
PI. II, A, from a photograph of a piece of red slate from the old quarries of the 
Fair Haven Red Slate Company, in the southeast corner of Whitehall, N. Y., shows 
a bed an inch thick crossing the cleavage diagonally, and therefore spreading out 
to double that width on the cleavage surface. In the center of the "ribbon" is a 
bed one-fourth inch thick of greenish limestone, and on either side of the ribbon is 
a very thin rim of green slate; the rest of it is purple. Under the microscope the 
composition of these little beds is this: The central green consists of calcite and sider- 
ite rhombs; some of the siderite is altered to limonite. There are large quartz grains, 
muscovite scales without parallel orientation, and occasional plagioclase grains. The 
purple consists chiefly of scales of muscovite and chlorite, lying in two directions, at 
right angles to each other, irregular dots of hematite, some carbonate rhombs, quartz 
grains, and rarely a grain of plagioclase. The thin green strips on the sides contain 
less hematite than the purple, and a large number of the muscovite and chlorite scales 
lie parallel to the bedding and transverse to the cleavage. The red slate itself is like 
the purple, but contains far more hematite and probably less chlorite. The iron 
obscures the other minerals. In this specimen the central bed of quartzose limestone 
is probably due to change of sediment. The varying amount of ferric oxide in the 
purple and green parts of the ribbon as compared with the red of the slate beds may 
be due to a chemical change brought about by the decomposition of organisms on the 
sea floor, as has been shown on pages 15-18 to be probably true of the green and 
purple spots in the same slate. Sometimes the quartzose ribbons of the red slate are 
parallel to the cleavage. This parallelism between bedding and cleavage character- 
izes some of the Maine and Vermont quarries (see PI. XII). 
The Vermont purple slate frequently has small beds of green slate, with or with- 
out a central band, crossing the cleavage; and the village sidewalks in that region 
are sometimes flagged with such purplish green-banded slate. When, owing to fold- 
ing and pitch, the grain of such slates is not at right angles to the bedding the roof- 
ing slates have diagonal green ribbons. These green ribbons, both in the red slate 
