STRUCTURE. 21 
of New York and the purple of Vermont, run into planes or rows of spots of various 
sizes, usually more or less oval in outline. When a series of such spots are in line it 
indicates the course of the bedding. 
The small beds or ribbons are often plicated, as in tigs. Q, R, PI. XX IV. PI. TV, 
E, is a microscopic drawing of a thin section of the plicated bed of PL XXIII, I). 
The bed here consists of calcite and vein quartz, the original calcareous sedimenl 
having been crystallized and vein quartz deposited. The diagram shows the fault- 
ing of the bed, the bending of the cleavage foliation, and the slip cleavage caused by 
the dislocation. The plication of such small beds of quartzose limestone and cal- 
careous quartzite is sometimes extreme, as shown in PI. IV, A, B, IT. The folding in 
such cases seems to have been preserved only in the hard beds, while in the more 
plastic material of the slate on either side slaty cleavage has obscured or effaced the 
bedding. a 
Fig. F, PI. IV, represents a plicated bed of quartz and one of calcite separating beds 
of purple and green slate. Under the microscope both quartz and calcite beds are 
bordered with chlorite scales on the outside, and separated by such scales; there is 
also some pyrite along the edges. The cleavage of the slate is at right angles to the 
course of the bedding, but is slightly deflected near the plicated beds. In several 
places the slaty material has been drawm partly into the bed. 
A thin section of a small plicated bed of quartzite in the purple slate of the Cedar 
Point slate quarry, in Castleton, Vt., shows the following: The bed consists mainly 
of quartzite, but this contains grains of plagioclase, rhombs of carbonate, probably 
calcite, and scales of muscovite. Toward the slate there are coarse fibers of muscovite. 
The slate merges into the quartzite, sending out long streamers of sericite, which 
penetrate between the grains of quartz and calcite. The slate contains large scales of 
chlorite within the meshes of sericite, which scales lie at right angles to the cleavage, 
i. e., parallel to the course of the bed. It also contains grains of quartz. The fibrous 
character of the slate is apparent at the border of the quartzite bed. The signifi- 
cance of such a bed is that sandy material was deposited for a brief interval during 
the deposition of the finer material which produced the slate; there were grains of 
quartz and of feldspar, and probably scales of mica, together with calcareous mud. 
Under the compression and the chemical changes which accompanied it the quartz 
grains were cemented into quartzite, the calcareous mud was crystallized, and the bed 
was plicated and became entangled with the slaty material. The slaty material itself was 
also somewhat plicated, and a secondary cleavage (slip cleavage) was produced in it. 
PL V, A, from the Ordovician syncline at West Castleton, also illustrates this plica- 
tion of quartzite beds and the overturning of the folds. Fig. T, on PL XXIV, shows 
these features on a larger scale. 
Exceptionally the quartzite beds seem to have been pushed out of their normal 
parallelism, even without folding or faulting. 
A case of brecciation on a somewhat large scale is shown in fig. G, PL IV, taken 
near fig. T, PL XXIV. 
Frequently, however, the bed surface is simply a parting whose meanderings must 
be carefully followed in order to distinguish it from fractures of various kinds, as at the 
West Pawlet syncline, PL XXV; or the bedding may be indicated by the weather- 
ing out of calcareous matter from the slate itself, some beds containing more of it 
than others, as at the syncline at West Castleton, PL III. The rock at Wesl 
Castleton is a shaly slate, consisting of alternating light and dark gray bands, 
i. e., beda of muscovite and chlorite scales, grains of quartz, spherules of pyrite, and 
some carbonate, but there is more carbonate in the gray bands than in the black ones, 
which contain more carbonaceous matter. Now and then there is a'minute bed con- 
«See C. H. Hitchcock, Second Ann. Kept. Geol. Maine, 1862, p. 285, fig. 45, where a bed of strongly 
plicated limestone lies between implicated beds of slate. See also the classic figure of H. C. Sorby given 
in his paper, On the origin of slaty cleavage: Edinb. New Phil. Jour., vol. 55, July, 1853, pp. 139, 140, 
which was reproduced by Tyndall in his Roy. Inst. Lecture, and also by Phillips in his British Assoc. 
Report on cleavage. 
