VERMONT. 
105 
measuring from 0.003 to 0.015 and even 0.05 mm. in diameter. These sometimes 
consist of two crystals, an inner rhomb and an outer one, with different orientation, 
possibly in twinned position. In some cases the central rhomb has fallen out, in 
others the rhomb has an opaque mineral nucelus which is not pyrite (magnetite 
or graphite?). There are also a few scales of chlorite with interleaved muscovite 
up to 0.13 mm., with their laminae across both grain and cleavage, and some equally 
large muscovite scales, possibly of sedimentary origin. There are some irregularly 
distributed pyrite spherules from 0.008 to 0.02 mm., also minute lenses of pyrite. 
Rutile needles, measuring from 0.0017 to 0.009, rarely 0.012, by 0.002 mm., average 
from 1,000 to 1,850 per square millimeters, which is equivalent to from 645,000 to 
1,200,000 per square inch; rarely a grain of zircon or prism of tourmaline 0.022 by 
0.008 mm. 
The chief constituents of this slate, arranged in descending order of abundance, 
appear to be muscovite (sericite), quartz, carbonate, chlorite, rutile, pyrite, magnetite. a 
In order to ascertain the cause of the discoloration thin sections across the discol- 
ored surface of a slate which had been exposed three years were examined and it was 
found that while the carbonate rhombs within the body of the slate were transparent 
in ordinary light those at the surface were changed to the color of limonite. These 
rhombs measured 0.047 mm. A cleavage surface of the same slate was also affixed 
to the glass slide and the other side was ground down. This showed a multitude of 
rhombs, generally ranging in size from 0.008 to 0.013 mm., entirely or partially 
altered to limonite. In some cases there was a yellowish brown zone of alteration 
surrounding an unaltered nucleus. These rhombs are regarded by Doctor Hille- 
brand as an isomorphous mixture of dolomite and siderite, i. e., a carbonate of 
lime, magnesia, and iron, in winch the iron (ferrous carbonate) oxidizes into limon- 
ite.^ The only way to prevent this discoloration would be to coat the slate with 
some preparation which would protect it from oxidation. 
Some of the sea-green slates are termed hard, others soft. The difference, judging 
from microscopic evidence, seems to be due to the greater percentage of carbonate in 
the soft ones and the larger size of the quartz grains in the hard ones. 
The following complete analyses of sea-green slate were made by Dr. W. V. Hille- 
brand, chemist of the United States Geological Survey: 
Analyses of Vermont "sea-green" roofing slate 
Si0 2 (silica) 
Ti0 2 (rutile; titanium dioxide). 
A1 2 3 (alumina) 
Fe 2 3 (ferric oxide) 
FeO (ferrous oxide) 
MnO (manganese oxide) 
H 2 (water below 110° C. 
67.76 
.71 
14.12 
.81 
4.71 
. 10 
NiO, CoO (nickelous and cobaltous oxide) Trace 
CaO (lime) 
BaO (baryta) 
MgO (magnesia ) 
K 2 (potassa) 
NaoO (soda) 
63 
' .04 
2.38 
3.52 
1 . 39 
Li 2 (lithia) Strong 
trace. 
.23 
62. 37 
.71 
15.43 
5.34 
.22 
Trace. 
.77 
.07 
;;. ll 
4. 20 
1.14 
Trace. 
.34 
59. 8 1 
.71 
15.02 
1.23 
4.73 
. 3 1 
Trace. 
2.20 
.09 
:;. ; i 
4.48 
1.12 
Strong 
trace. 
a For colored lithographs of magnified thin sections of this slate as seen under both ordinary and 
polarized light, see Nineteenth Ann. Rept. U. S. Geol. Survey, pt. 3, PI. XXXV. 
''See Doctor Hillebrand's Chemical notes on the composition of the rooting slates of eastern New 
York and western Vermont, p. 39. 
