162 A. WICHMANN ON A MICROSCOPICAL STUDY 
cimens is by far too small to fix such a law, especially if we con- 
sider that even among these there are some exceptions. G. R. 
Credner examined only two specimens of clay from the Tertiary 
period. He found in clay from the Isle of Wight very numerous 
microlites, whilst in a Tertiary clay from Dolau, near Halle, Germany, 
he only observed very few and minute crystalline constituents. At 
the same time we must add that it is very difficult to decide on the 
proportion of the crystalline constituents in clay-slate and slate- 
clay. 
ii we agree that this statement is correct, it follows that the older 
sedimentary rocks are the richer they must be in crystalline consti- 
tuents, till we finally conclude that a period must have existed in 
which the secretions of the ocean were so great that the production 
of crystalline constituents must have been due to them; and it is at 
this that G. R. Credner evidently aims. In this manner, also, the 
important question respecting the origin of crystalline schists would 
be solved. We should like first to point out that the secretory 
capability of the ocean requires confirmation ; for up to the present 
time no proof of this property has been given, and therefore the idea 
cannot be admitted into the discussion. Nor has it been proved that 
any change has taken place in the chemical composition of the ocean 
during the course of geological periods. Thus, for instance, the clay- 
slate of 8. 28, T. 39, R. 18, Wisconsin, is regularly deposited be- 
tween chlorite-schist and actinolite-schist, so that, according to G. R. 
Credner’s theory, the ocean held successively in solution or suspension 
first chlorite, then clay, then actinolite, a fact which certainly has 
not been proved. 
It has also been demonstrated by the above-mentioned investiga- 
tions that the proportion of crystalline constituents by no means 
coincides with this theory. If some clay-slates are extraordinarily 
rich in crystalline constituents there are others which contain but 
small quantities, and consequently we have no means of correctly 
ascertaining the proportion. Still greater force is given to this 
argument by the fact that we made more numerous examinations of 
the Huronian clay-slates from Michigan and Wisconsin than G. R. 
Credner did of those from the Carboniferous period down to the 
Diluvium. 
(3) **These crystalline structures have not originated in conse- 
quence of any later metamorphic actions whatever upon the formed 
rock ; they rather owe their origin (as proved by 
a. Their position parallel to the bedding-planes, and 
b. Their not unfrequent radial grouping around a clastic 
fragment of rock which serves as a nucleus) to a 
primary segregation from the same waters from which 
mechanically transported mineral particles were simul- 
taneously deposited to form muddy sediment.” 
These supposed proofs can on no account be permitted to pass as such; 
for the deposition parallel to the slate-plane may be explained by either 
theory. For whether the crystalline constituents be a product of the 
secretion of the ocean, or whether they haye been formed in the still 
