274 Transactions of the Royal Microscopical Society. 
was no other than quartz finely divided and dispersed through 
the stone. My researches prove, however, that they contain 
scarcely any quartz. Hence it is to the garnets that we are to 
attribute this hardness of the rock, for we know that the specific 
hardness of garnets is comprised between 6°5 and 7°5 of the scale 
of Mohs. The discovery of garnets in the slates of Recht by my 
friend Dr. Zirkel, Professor at the University of Leipsic, is another 
support of this interpretation, as the slates of Recht belong to the 
same formation as the rocks of Salm, of which they are the con- 
tinuation in Germany. 
The largest garnets of our preparations show inclusions which 
are found so often in this mineral when studied under the micro- 
scope; we observe also in the largest those irregular fissures which 
are so characteristic of garnet. 
But how is the yellow colour of the whetstone explained if we 
admit that this rock is, as we have said, almost exclusively com- 
posed of garnet? In our endeavours to explain this colouring, we 
have succeeded in determining the sort of garnet to which this mineral 
belongs. We place it in the variety called Spessartine, and we shall 
soon see that another kind of proof confirms us in this classification. 
In considering spessartine as the principal element of the 
whetstone, we see that the union of a very great number of 
infinitely small crystals of this mineral should produce, when 
regarded ‘ ensemble,” a yellowish-white tint; for the purest 
spessartine is in little transparent crystals of a pale yellow, im the 
island of Elba, at St. Marcel, in the diamond sands of Brazil, 
and in Maine in the United States. We thus easily understand, 
in admitting that spessartine is present here, how an agglo- 
meration of small garnets of this variety can produce the yellow 
tint of the whetstone. But what indicates still more certainly the 
manganese garnet is chemical analysis. M. von-der-Mark has found 
as much as 21-71 per cent. of MnO, and M. Pufal has found 
17°54 per cent. in a specimen he has had the kindness to analyze 
for us. This large quantity of manganese should not cause sur- 
prise, since it is known that the spessartine of Haddam, in Connec- 
ticut, analyzed by Rammelsberg, gave 33 per cent. of MnO. Let 
us also bear in mind that the presence of manganese manifests 
itself in a remarkable manner when tested by a bead of borax. 
Let us also remark that all the surrounding rocks are as it were 
impregnated with manganese; it is found in the form of veins, or 
combined in the remarkable metamorphic minerals (ottrelite, de- 
walquite, &c.) of that region. 
And it is near veins of whetstone that MM. de Koninck and 
Davreux discovered the beautiful little garnet crystals of which they 
have given the description,* and whose composition approaches 
* *Bulletins de PAcadémie Royale de Belgique, 1873. 
