On the Belgian Whetstones. By Rev. A. Rénard. 275 
more nearly that of the typical variety of spessartine than that of 
any specimen of which an analysis has been published up to the 
present. I may add, that some specimens of whetstone have lost 
their colour through an impregnation of hydroxid of manganese. 
These became completely black, but in thin sections there imme- 
diately appeared all the essential elements of the rock, coloured, 
however, a faint brown by the foreign matter. 
A third element of the whetstone of Salm is schorl. This 
mineral is far less widely spread than the two which I have just 
made known. ‘The principal microscopic characters which this 
mineral offers in the rock are the following: the form of the 
sections is that of a parallelogram whose great axis may have on an 
average from 0:07 mm.to 0'08mm. In width it reaches 0°01 mm. 
Ordinarily the sections of the mineral are terminated at one extre- 
mity by planes intersecting each other at an angle more or less 
open; the opposite side being terminated by an almost straight 
line. They are traversed by crevices which are sensibly parallel to 
the base, and often appear notched. Their tint is not homogeneous ; 
it is pale green, blue, greyish, sometimes growing stronger at one 
extremity of the section. ‘This mineral is birefringent, and is 
strongly dicroscopic. The sections are often filled with black and 
opaque little spots. It is known, moreover, that M. Angor has 
discovered this mineral in a great number of slates and schists. 
We find again in our thin section a most perfect resemblance 
between the mineral he considers to be schorl and those which 
we have been led to consider as the same. The forms of that 
mineral, we have said, show a difference of development for 
the two extremities: this difference is the very same as that 
which affects schorl; which presents us so frequently with the 
most classic examples of enantiomorphism. We have said that 
certain sections show two different tints at the two extremities; 
this difference of tint is a well-known fact in the case of this 
mineral. Indeed, we know that the transparency of the tourmaline 
varies in the same crystal with the direction relative to the axis of 
the rhombohedron. It is more sensible in a perpendicular, and 
feebler in a parallel section, a fact which must be referred to its 
dicroscopism. But besides this phenomenon it is not uncommon 
to find that the same crystal presents different colours at the two 
extremities, or at least a tint of unequal intensity. According to 
M. von Lasaulx, the same thing is noticeable in the microscopic 
tourmalines enclosed in the garnets of the granulites of Saxony. 
We have just said that these crystals of schorl sometimes appeared 
as if broken; that they had undergone certain deformations. We 
remark, indeed, that these small sections of schorl appear broken, 
and that their various fragments lie at a short distance. M. Rosen- 
busch observes that those which are seen by the microscope are 
