OF SOME HURONIAN CLAY-SLATES. 161 
varieties of clay-slates the so-called clay-slate needles are not present. 
The crystalline constituents are chiefly tourmaline, hematite, and 
quartz. ‘The other constituents of the clay-slate, as well as their 
structure, agree in general with former investigations. In novaculite 
garnets are present. In the carbonaceous slates and shales crystal- 
line constituents seldom appear. The former existence of organisms 
may be inferred from the presence of coal. 
D. Probable origin of the Crystalline Constituents in Clay-slate. 
Haying studied the structure and the different constituents of our 
clay-slates, we must now consider the probable origin of the crys- 
talline minerals. Three possibilities of origin may be accepted :— 
1. The crystalline constituents may be regarded as the produce of 
the chemical action of the ocean. 
2. They may be attributed to processes of metamorphism which 
took place after the solidification of these rocks. 
3. They may have been formed after the deposition of clay-slate 
mud, whilst this was still in a plastic state. 
The supposition that the crystalline constituents already existed 
in the rocks which supplied the material for the formation of clay- 
slate will scarcely find an advocate. The formation of tourmaline 
with its distinct outlines, being in complete contrast to that of 
the clastic constituents, contradicts such a theory. The crystalline 
nature being recognized by all authors, such a possibility need not 
be brought under consideration. 
1. We find a complete statement of the first theory in a paper by 
G. R. Creduer*. It is very important that the propositions adduced 
in it should undergo a minute examination, because from them the 
most important consequences ensue; whilst we must acknowledge 
that in accepting generally this principle, we have come much nearer 
the explanation of the origin of the crystalline schists. The results 
of the paper of G. R. Credner are contained in the following propo- 
sitions} :— 
(1) “ Crystalline products of segregation, such as Zirkel has de- 
monstrated in the Silurian and Devonian clay- and roofing-slates 
side by side with their clastic constituents are not confined to the 
above-mentioned rocks of the two oldest Palzozoic formations, but 
rather form a more or less essential constituent of every slate- and 
elay-rock hitherto examined of all (even the most recent) periods.” 
(2) “ In general, in the rocks investigated, the part taken by these 
crystalline segregations in the formation of the rock decreases pari 
passu with the geological age. A Mesozoic clay-rock consequently 
consists of far more clastic and less crystalline material than a Pa- 
leozoic one. Carboniferous slates stand in the same relation to 
those of the Deyonian and Silurian. Isolated exceptional cases may 
have local causes.” 
We do not readily agree io this. The number of examined spe- 
* Zeitschrift f. d. ges. Naturw. 1874, p. 505. t Ibid. p. 522. 
Q.J.G.8. No. 137. M 
