156 A. WICHMANN ON A MICROSCOPICAL STUDY 
11. A Microscoprcan Srupy of some Hurontan Cray-sLaTEs. 
By Dr. Arrnur Wichmann. (Read June 19, 1878.) 
[Communicated by the President. | 
Introduction. 
GREATER attention has lately been paid to the microscopical study of 
‘clay-slates and slate-clays, giving rise to a number of researches 
the results of which may be found in the classical paper of Zirkel*. 
The principal object of these investigations has been to discuss the 
origin and formation of the crystalline constituents contained in 
clay-slates, and they have chiefly extended from Silurian and De- 
vonian to rocks of the recent period. Such clay-slates as belong to 
Archean districts might also claim the privilege of a certain in- 
terest, especially as the presence of these crystalline constituents 
has been used to solve geological problems. ‘The clay-slates which 
I have examined occur in the Huronian region of Lake Superior. 
The respective rocks are in the collection of Major T. B. Brooks, 
Marquette, Michigan, who placed them at my disposal, chiefly for 
the sake of microscopical examination. 
Hermann Credner in his papery, ‘‘ Die vorsilurischen Gebilde der 
‘Oberen Halbinsel’ von Michigan, in Nord-Amerika,” gives us a 
partial description of the geological appearance of these rocks. 
Further information may be obtained from the Geological Survey of 
Michigant, to which we shall refer hereafter. 
Those clay-slates which appear in the Huronian region are found 
to be separate strata, deposited on the upper strata of quartzite, ex- 
tending, as Herm. Creduer states §, to a depth of 8500 feet, and form- 
ing layers between the quartzite and dolomite. They chiefly repre- 
sent true clay-slates, not different in appearance from those of other 
geological periods. Subordinate strata are formed by hard brittle 
novaculite (whetstone), of a yellowish-grey colour. Carbonaceous 
slates and shales are generally found in black masses, and may 
be added to the slate-clays. 
The following Table by T. B. Brooks and Herm. Credner shows the 
relations and classification of the clay-slate strata. 
* Poge. Ann. exliy. 1871, p. 319. 
+ Zeitschrift d. deutschen geol. Ges. 1869, p. 534. 
{ Michigan Geolog. Survey, vol. i. & ii. (New York). § Loe. cit. p. 553. 
