94 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 
The Lower Cambrian slates of western Vermont are greenish gray, purplish, and 
"variegated," i. e., greenish gray and purplish mixed. These occur in alternations. 
The quarry diagrams (Pis. XXIII and XXIV) show that there is little regularity in 
these alternations. In the main, however, this horizon seems to consist of from 100 
to 140 feet or even 200 feet of greenish and purplish slates, the greenish ones predom- 
inating, with from 40 to 50 feet of variegated or mottled overlying, but possibly 
replacing the purple in places. On the west side of Lake Bomoseen nearly 100 feet 
of purple are exposed. The purple sometimes contains a few inches of dark-reddish 
slate not unlike the red of the ( )rdovician. There is some difference in the shade of 
the different beds of green in the same quarry, some being more greenish, others 
more grayish. There are also differences in the amount of discoloration produced 
by weathering in beds of the same locality. Although some quarries produce only 
the so-called ''unfading green"' and others only the "sea green," these differences 
appear not to belong to strata of different ages, but to occur at different points 'in 
strata of the same age. 
Interbedded with the slates are beds of calcareous quartzite of very different areas, 
ranging in thickness from a few inches up to 5 feet. This quartzite contains a few 
grains of plagioclase and more muscovite scales, and is veined with quartz, which' 
crystallizes in cavities. The quartzite sometimes weathers brown; its calcite, there- 
fore, probably contains some siderite. 
Associated with the slates are also beds of limestone conglomerate or breccia rang- 
ing from a i«'\\ feel to I" feel in thickness, carrying the trilobite Olenellus and other 3 
fossils characteristic of the Lower ( Jambrian. < me of these beds of limestone breccia 
is of frequenl occurrence in the quarries, overlying the slate. (See quarry diagram 
/', PI. XXIV.) 
The slate-bed surfaces are generally covered with annelid trails or impressions oi 
algae, or both. The purple slates are often ribboned or handed with light-green slate 
beds an inch or more in thickness, or have oval or roundish light-green spots, fre- 
quently in rows. Similarly the "sea-green" slates have grayish ribbons crossinj 
them. 
• The position of these commercial slates in the Lower Cambrian series is shown ir 
the following table, in which the beds are arranged in their natural order: 
Loim r Cambrian rocks of slate belt of western Vermont. 
Feet. 
(E) Ferruginous quartzite and sandstone 25-1(1 
(D) Black shale and slate with Lower ( 'ambrian fossils 50-23 
(C) Black patch grit; a dark-gray grit or sandstone with black shaly 
patches, b (times with calcareous nodules carrying Lower Cam- 
brian fossils 10-41 
(B) Roofing slate, grayish green, purplish or mixed green and purplish, 
alternating with beds of calcareous quartzite and limestone breccia 
up to 40 feet thick, the latter carrying Lower (ambrian fossils 200-24 
(A) Olive grit or graywacke, more or less massive, with minute scales of 
hematite or graphite, sometimes with small quartzite beds, frequently 
calcareous, generally weathering a pale brick red. Under the micro- 
scope shows grains of quartz, feldspar, large scales oi muscovite and 
of chlorite interleaved with muscovite. Associated with this isabed 
of quartzite 12-55 feel thick. 50-20 
The base of the formation is not certainly exposed. There may be a bed of con ■ 
mercial slate in the olive grit. 
The probable structure and structural relations of the two slate formation- <' 
western Vermont are shown in PI. XXII. Each section is described beyond i 
detail. Those parts of the sections which are well substantiated are indicate! !>• 
