OF SOME HURONIAN CLAY-SLATES. 159 
quantities in thin sections which come from the same locality, but 
it is always present. 
The occurrence of narrow, rod-like, colourless crystals in some 
specimens is very striking; but being very small, they cannot be 
identified with a distinct mineral. It may be that they belong to 
the class of felspars. On an average they are 0:01 millim. in 
length. ‘They are deposited parallel to the slate-plane, and appear 
one-coloured by polarized light. Specimens with distinct crystals 
occur in the Chocolate-Marble Quarry, Michigan ; they occur also 
very abundantly in the novaculites which are yet to be described. 
Mica can be recognized as a crystalline constituent chiefly when 
it is found in radiating fragments ; otherwise it is difficult to recog- 
nize it as either a clastic or a crystalline constituent. It is found 
in almost colourless thin lamine, which often exhibit a fibrous struc- 
ture like sericite. Many attempts have been made to identify these 
lamin with talc; but to justify this, magnesia ought to be contained 
in greater quantity in these rocks. It is very remarkable that the 
mica-laminz should be only sometimes deposited parallel to the 
slate-plane ; such is the case with a clay-slate near centre of 8S. 13, 
T. 50, R. 32, Michigan. 
Some parts of the ground-mass show a crystalline structure. This 
is only to be seen by polarized light, because these parts consist of 
an aggregation of irregular spots, bluish and bluish-grey in colour, 
and vanish by degrees into the amorphous ground-mass. It is pos- 
sible that these parts represent either quartz or a crystalline silicate. 
Calcite cannot be considered a constituent of these clay-slates, 
although thin veins of this mineral are found interspersed within 
the rocks. 
T. B. Brooks* mentions a clay-slate containing garnet, which was 
discovered at Champion Mine, Michigan. 
Zirkel? has not ascertained whether the colourless ground-mass, 
which represents the cement of all constituents, is opal or a porodine- 
amorphous silicate. Some time agot we expressed our opinion 
that it might be a porodine-amorphous silicate, amorphous quartz of 
ante-Tertiary age not being known. Besides, clay-slate is charged 
with too little silica (50 to 64 per cent.) to encourage us to consider 
the cement as opal. 
B. Novaculite ( Whetstone). 
Novaculite is a very hard brittle clay-slate, generally of a yellow- 
ish-grey colour. The abundant presence of crystalline aggregates 
in contrast with the decreased amorphous ground-mass is proved by 
the microscope. These aggregations, which account for the well- 
known hardness of this slate, consist chiefly of quartz. Besides 
these, various kinds of crystals and fragments of towrmaline are 
found, the latter also appearing in form of microlites. Blood-red 
lamine of hematite, often possessing a distinct hexagonal form, are 
* Loe. cit. 
+ Mikroskop. Beschaff, p. 493 (1873). t N. Jahrb. f. Min, 1876, p. 917. 
