OF SOME HURONIAN CLAY-SLATES. 163 
plastic mud, or owe their origin to metamorphic influences, they will 
in general occupy such a position either by the pressure of the super- 
incumbent layers, or, if they were formed after the settling of the 
rocks, because parallel to the slate-plane they find the least relative 
resistance. The radiating grouping can still less pass as proof. Even 
if we allow the possibility that while a fragment is sinking in the 
ocean, it can unite with other minerals which become crystallized, 
it must be clear to every one that the motion of the water would very 
quickly separate them again. According to G. R. Credner’s own 
statement it was impossible in some of his preparations to discover 
the manner of accretion of any crystalline formations, because the 
particles have been again separated by the action of water during the 
preparation of the object. When even in a glass of water such me- 
chanical effects are apparent, how much more must they become so 
in the ocean ? 
2. We might seek a further explanation of the origin of these 
crystalline constituents in metamorphic processes, which possibly 
had some influence after the solidification of the clay-slate mud. 
This origin is especially urged by Delesse*. The chief objection 
to this theory is found in the existence of broken and reunited 
particles. The being broken per se would not contradict metamor- 
phism, were it not that the material which reunites the fragments 
is composed of the same clastic clay-slate substance. 
3. This aids in the confirmation of a third theory, which supposes 
the formation of the crystalline constituents to have taken place 
while the rocks were still plastic. This view is still further sup- 
ported by the fact that new-formed materials are discovered in groups 
round some fragments. According to his close examination, Zirkel 
first proposed the theory that the crystalline constituents either had 
been formed during the deposition of the mud or at least before its 
solidification. More recently Svedmark, in his paper on Cambrian 
clay-slate, has stated that the garnets were formed whilst the clay- 
slate was still soft and plastic. Sorby* also states that the green 
grains of glauconite ought rather to be attributed to chemical action 
occurring either during or soon after deposition, like the minute 
erystals met with in some of the dredgings from the Pacific ocean. 
As the result of our investigations this opinion may be confirmed f. 
Later metamorphic processes are not to be excluded. We would 
especially draw attention here to the fact that chlorite-, mica-, 
hornblende-, sericite-, and other schists contain tourmaline and 
hematite of the same appearance as true clay-slates. It is probable 
that these minerals existed in the crystalline schists before the origin 
of the rest. If we want to place the metamorphic processes in their | 
* Revue de Géologie dans les années 1873 et 1874 (Paris, 1876), p. 203. 
+ Monthly Microsc. Journ., Feb. 1877. : 
} I regret to have received the interesting paper of Renard on the Belgian 
Whetstones (‘ Mémoire sur la structure et la composition du coticule, Bruxelles , 
1877) after thie paper was already finished. Renard mentions also the abun- 
dance of tourmaline and garnet in these rocks, and has also expressed the opinion 
that these constituents were formed during the deposition of the sediments (loc, 
cit. p. 38). 
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