164 ON A MICROSCOPICAL STUDY OF HURONIAN CLAY-SLATES. 
proper place, we should do so in ranging them as a fourth phase 
in the production of the actual condition of sedimentary rocks :— 
1. Deposition of mud. 
2. Formation of minerals during the plastic state. 
3. Formation of rock as clay-slate (solidification of the ma- 
terial). 
4, Metamorphic processes. 
The organic remains which had heen deposited with the mud 
caused the formation of the carbonaceous particles. They might 
possibly have caused also the formation of pyrites by reduction of 
solutions of sulphate of iron. 
There is one mineral occupying a strange position with regard to 
the occurrence and the formation of the other crystalline constituents. 
It does not appear in distinct crystals, but forms aggregations which 
are only to be observed with polarized light, and vanish by degrees 
into the amorphous ground-mass. It is as possible that it represents 
a silicate as that it is quartz. It seems to have been formed during 
the solidification of the rock, and is similar in appearance to the 
ground-mass of some crystalline schists. 
It is only by comparing crystalline, semicrystalline, and clastic 
sedimentary rocks that we shall arrive by degrees at a satisfactory 
explanation of the origin of the crystalline schists which make up 
the greater part of the Archean formations. 
