272 Transactions of the Royal Microscopical Society. 
number of particles, more or less circular, scattered along the surface 
of the filaments. The elongated microliths are in line, and grouped 
with their greater axis parallel to the direction of the micaceous 
lamine. Clearer regions are also remarked in this mass, almost 
without interpositions, the arrangement of which contrasts with the 
general disposition of the elements. These veins, having a thickness 
of less than a millimeter, advance irregularly across the rock. They 
are made up chiefly of micaceous substance, and do not seem to be 
fissures filled up later on, but rather to have been formed at the 
same time that the rock took its distinctive petrographic character. 
They are found to contain, though in small proportions, all the 
elements which we discover as constituent parts of the whetstone. 
A fissure, filled later on, would not be filled with all the substances 
identical with those which form the rock. This kind of micro- 
scopical primary veins does not cross the whole of the thin section. 
They often form lenticular regions in it, of the same structure and 
composition as the little veins of which we have just spoken. 
None of the microscopical characters by themselves allow of a cer- 
tain identification of the lamelle with a determined phyllite. I give 
up, however, the opinion of Dumont, who considered, without sufli- 
cient proof, pyrophyllite to be the constituent principle of the phyllas 
of the Ardennes ; and I would bring these phyllitous fibres into con- 
nection with Damourite, a mineral which the chemical analyses 
of Messrs. Davreux and de Koninck, jun., have shown to exist 
in a garnet-bearing rock of Salm, that presents, as will be seen, 
many analogies with the whetstone. Still it is very difficult to 
decide this point with certainty, since it is impossible to isolate 
the lamellae for a separate analysis, and they are dotted with foreign 
minerals. On the other hand, the optical reactions are unsatis- 
factory ; for, as is known, one of the problems the most difficult of 
solution in the microscopical analysis of rocks is the specific deter- 
mination of the different micas, especially when they are found 
without distinguishable crystalline forms, and with extremely small 
filaments, as is here the case. 
Let us now pass to a more minute study of the different elements 
contained in that rock. For this we must use a magnifying power 
of from 400 to 600 diameters. With such aid we may perceive 
the innumerable granules, scattered over the surface of the lamine, 
become clearly and distinctly individualized, and the rock, at certain 
points, appears to be composed of globular forms whose agglomera- 
tion almost completely veils the micaceous element. The mean 
dimensions of these circular forms rarely exceed 0°02 mm. ; and, 
according to an approximative calculation, some parts are so beset 
with them, that a millimeter cube of rock contains over 100,000. 
These globules, for the most part rounded, sometimes also elongated, 
are, in some cases, terminated by crystallographic forms. They 
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