On the Belgian Whetstones. By Rev. A. Rénard. 271 
be answered by macroscopical examination alone, since the compact- 
ness of the rock and the fineness of its grain is such that, even with 
the aid of the magnifying glass, it is impossible to individualize the 
mineral species which make up the whetstone. It appears in 
general as a homogeneous substance of clear colour, and the geolo- 
gists who have occupied themselves with this rock have limited 
themselves to giving the description of some of its physical pro- 
perties and the details relative to its bearing, touching only inci- 
dentally on its composition. This, however, is not surprising when 
it is remembered that these skilful observers had not at their 
disposal microscopical analysis, which I have been able to use. 
Still, on examining with attention the lamine or the fractures, 
peculiarities may be seen at their surface which give a glimpse of 
the elements that microscopical analysis makes known. With the 
naked eye phylladic plates, closely aggregated, may be perceived. 
This phyllite has not the silvery or pearly aspect of sericite ; 
neither has its pyrognostic characters. It is these lamella which 
make up the fundamental mass of the rock. By the reflexion of a 
strong light there may be seen on these phylladic membranes 
a glistening due to crystalline granules of infinitesimal dimensions. 
These shining grains would naturally be attributed to quartz, were 
it not that study at the microscope discovers in them optical properties 
and crystalline forms which put aside this supposition. The micro- 
scopical dimensions of the elements mingled with the phyllite never 
determine the structure which we have called by the name of 
gneissic in our study of the rocks of the French Ardennes; at 
least, it does not appear to the naked eye nor under the magni- 
fying glass. Among the accidental elements visible to the naked 
eye should be counted iron glance, hydroxid of manganese, which 
often impregnates whetstone, quartz, and pyrophyllite. 
Let us pass now to the study of the ultimate constitution 
of the whetstone, as shown by microscopical analysis. In order 
to come at the results given in this paper, I have cut and polished 
more than seventy thin sections of this rock. ‘To have a general 
idea of the ultimate structure of the whetstone, we should study 
it first with the help of weak magnifying powers. There is 
then perceived a colourless micaceous substance which seems to 
make up in great measure the fundamental mass. These phylladie 
fibres, generally elongated and colourless, appear peppered with a 
black dotting, owing to a numberless quantity of granules which 
have the appearance of opaque points. There are seen also in- 
numerable microliths, some like simple dashes, and others of larger 
dimensions, but much more rare, showing a light bluish tint. 
Finally, with the help of a polarizing apparatus, it may be seen 
that one part of the mass belongs to an isotropic substance. 
Commonly the phyllite is completely covered by the prodigious 
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