158 A. WICHMANN ON A MICROSCOPICAL STUDY 
usual colour is dark brown or black, they sometimes appear of a 
faint red tint. Their specific gravity is on the average 2°75. 
Our microscopical investigations have also proved a composition 
corresponding with that of clay-slates of other formations. We at 
first perceive a colourless isotropic ground-mass, in which the other con- 
stituents are apparently imbedded, whilst throughout are found dust- 
like particles of a deep grey colour, which represent the chief con- 
stituent, and consist probably of clay-substances, the greater part of 
them probably of kaolin. Sorby* has adverted to the fact that the 
extremely minute granules of kaolin of clays, when mounted in 
Canada balsam, may be almost or quite invisible (these two sub- 
stances having nearly the same indices of refraction), whereas 
when examined in water there is no difficulty in recognizing these 
granules. Now and then quartz occurs in the form of small frag- 
ments, which are easily to be recognized by polarized light. 
Felspar is very rarely seen, and even then only in very indistinct 
fragments, whilst mica is more frequently found. Meddish-brown 
particles of hydrated oaide of iron are not unfrequent. The black 
flakes, which appear very often, consist of coal; they disappear 
when the clay-slate is placed before the blowpipe-flame. A difference 
is observed in the proportion of the crystalline constituents. The 
microlites of an unknown mineralogical nature called “ clay-slate 
needles,” which appear in great numbers in the German clay-slates, 
are here entirely absent. 
Besides a few other minerals, hematite and tourmaline frequently 
appear. Most beautiful individuals of tourmaline are found in a 
clay-slate (S. 32, T. 40, R. 36, Wisconsin T) as perfect crystals, as well 
as in fragments, 0:015-0:06 millim. in length, and 0:003-0°025 
millim. in breadth. In accordance with the investigations of Anger ¢ 
and Svedmark §, we may state that a comparatively frequent hemi- 
morphism of crystals occurs; their colour is greyish green, and they 
show a strong dichroism when tested with a single Nicol’s prism. 
Crystalline fragments are often found near one another, so that the 
original crystal can be reconstructed in imagination without any 
difficulty. Tourmaline is also found in exquisite crystals, as well as 
in fragments, in other clay-slates from Wisconsin (8. 28, T. 39, R. 18, 
and 8. 14, T. 40, R. 18). This mineral occurs less frequently in 
the clay-slates of Michigan ; and in some specimens from the Cho- 
colate-Marble Quarry, Marquette, it was entirely absent. Besides 
tourmaline, small yellowish-brown and red transparent crystals, 
which remind us vividly of zircon, as described by Zirkel||, were per- 
ceived in clay-slate from Slate River, Michigan. 
Hematite appears in exceedingly small, well-shaped, hexagonal 
laminee. Some are transparent, of a blood-red colour, but the 
greater number are opaque. This mineral also occurs in varying 
* Microscop. Journ., March 1877. 
t+ These numbers indicate the localities from which the specimens were 
derived, as marked upon the Survey Maps. 
{ Tschermak, Mineralog. Mittheil. 1875, p. 162. 
§ Geol. Forening. Stockholm, 1877, No. 38. 
|| Neues Jahrb. f. Mineralog. ete., 1875, p. 628. 
