On the Belgian Whetstones. By Rev. A. Rénard. 279 
This interpretation explains the heart, or geniculated, twins which 
are presented to us in the little prisms contained in almost every 
thin section of the whetstone, and of the regular groupings of the 
rocks of Sart. 
In spite of all the details resulting from a minute study of 
these remarkable crystals, we must, however, confess that they are 
not sufficient to determine to what species of mineral they belong. 
We are here before one of the most difficult problems of petro- 
graphy, that of identifying with a macroscopic species, crystals of 
such small dimensions as those which we have discovered, and 
which besides show characteristics which seem to bring them near 
to known minerals. Yet there is nothing to prove that these 
microliths are not a new species. The abundance of material 
that we have at hand, and the extraordinary development of these 
forms in certain specimens that we have endeavoured to analyze 
with a scrupulous care, authorize us to think that they ought not to 
be referred to augite ; in truth, epidote offers points of resemblance 
for the colour and the twin; for we know that Kokscharow has 
found hemitropies of epidote resembling these we have here ; but 
the other characteristics of epidote are too different to permit us to 
ascribe to it these little twinned prisms. 
In order to dissipate the doubts raised on this point, I have 
examined the analogous forms given us by the mineralogists for 
the minerals of the class of silicates, and I have been struck with 
the resemblance between the twins of chrysoberyl and our little 
twin crystals. What, besides, adds a certain value to this con- 
sideration is, that chrysoberyl is found in crystalline schist, as, for 
example, at Tarakoja in the Ural Mountains, and Marschen in 
Moravia. Stratigraphical and petrographical studies, moreover, 
lead us in like manner to consider the whetstone that we are 
describing as belonging to the series that we designate as crystal- 
line schists. 
In terminating this micrographical part of the article relative 
to the whetstone, let me remark that the oligiste iron appears 
but rarely, and even then sporadically, in this rock, and that it 
appears more generally, and especially, in contact with oligistiferous 
slate, whose relations with the rock we have just described we will 
now briefly study. 
At the commencement of this paper I drew attention to the 
fact that the whetstone appears almost constantly associated with 
the oligistiferous phyllade, in which it forms regularly intercalated 
strata. We have seen that these two rocks are perfectly adherent, 
as also that the foliation of the one is common to the other; now 
it remains to inquire whether their mineralogical composition and 
micro-structure cannot throw some light on the relations that exist 
between them. 
Let us see therefore what data the microscopical analysis gives 
