160 A. WICHMANN ON A MICROSCOPICAL STUDY 
not unfrequent. Colourless folia of mica are deposited, mostly parallel 
to the slate-plane. GARNET appears, in addition to these minerals, in 
more or less abundance. It has been already discovered by Zirkel 
in a noyaculite from Recht, in the Ardennes, and also by Svedmark 
in a Cambrian clay-slate from Lemmingtorp, in Hastgotland. The 
comparatwely great hardness of novaculite may be accounted for partly 
by the presence of garnet. This mineral generally appears in the 
form of rounded grains, seldom in distinctly defined crystals (rhom- 
bic dodecahedra). They are easily recognized, being colourless or of 
a yellowish colour, and possessing a rough surface. The garnets are 
much fissured, mostly free from any inclusions, and are recognized 
as perfectly isotropic bodies when seen under crossed Nicols. The 
above-mentioned narrow, rod-like, colourless crystals also appear in 
noyaculite. The crystalline aggregations partly consist of quartz; 
partly they may represent an indeterminate silicate. The deep- 
grey dust-like substance is also present, but in much less quantity 
than in true clay-slate. 
Typical specimens occur in Whetstone Quarry, Teal Lake, and 
Chocolate-Marble Quarry, Michigan. 
C. Carbonaceous Shales and Slates. 
There is little in these rocks to repay microscopical examination, 
but in a geological point of view they are of the greatest importance 
in the Huronian system. ‘They are black slates aud shales, which 
are proved before the blowpipe to contain considerable quantities of 
coal. In some cases the amount is found to be 22°51 per cent.* We 
can only account for their existence by the destruction and disinte- 
gration of some former organic substance which had been deposited 
with the clay mud at the bottom of the sea. This may be done 
by the same reasoning by which we trace the presence of coal in 
later periods. It can also be proved that, at least in the upper strata 
of the Archeean formation, organisms must have existed. Julieny be- 
lieves that he has found even a fucoidal impression in such a slate. 
Their spec. gravity, 2:06, is considerably lower than that of true 
clay-slate. 
In thin sections black opaque particles of coal form the largest 
proportion; they are of quite irregular forms and mostly grouped 
together ; small flakes of coal are also found in abundance in the 
amorphous clay-slate substance. This is especially the case in some 
specimens from S.E. Smith Mine, Marquette, Michigan. <A peculiar 
arrangement of the coal-particles is seen in a slate fromS. 11, T. 39, 
R. 19, Wisconsin ; they have been deposited parallel to the slate- 
plane in such a manner that one layer consists chiefly of coal-par- 
ticles. The slate containing most coal is that from L’Anse Iron 
Range, Michigan. Besides the constituents already mentioned, 
fragmentary folia of colourless mica sometimes occur. Pyrites here 
and there is not of unfrequent occurrence. 
It is proved by these examinations that in the before-mentioned 
* Mich. Geol. Survey, vol. i. p. 116. + Lbid. yol.ii. p. 5 
