On the Belgian Whetstones. By Rev. A. Rénard. 277 
given of the simple crystals of this kind. We find, however, in our 
sections, remarkable examples of grouping and of twins, of which 
we shall speak more in detail. 
When there is a certain quantity of these microliths gathered 
together, one is sure to remark, for some among them, a certain 
manner of adhesion or of superposition, which is too regular and 
constant in its repetition not to be subject to some crystallographic 
law (Fig. a). 
Wy 
ha 
Among these little crystals those of simpler form show the 
geniculated twins with an angle of about 60°, and in general it is 
with this angle that the crossings or superpositions of the minute 
prisms take “place. Oftentimes it isa granule of spessartine that 
serves asa point of attachment. The prisms do not preserve the 
same thickness through their entire length. In the upper part, for 
example, they appear a simple line, and toward the middle they 
suddenly bulge or swell out. Finally, im many cases they give 
rise, by their ramified disposition, to forms that may be considered 
as the skeletons of crystals that I have discovered in the whetstone 
of Sart, and of which I am now about to speak. 
In the thin sections of this whetstone there are to be observed 
triangular compound crystals, yellow, less transparent, and whose 
dimensions reach even to 0-05 mm. (Fig. b). These groups may be 
reduced to a single fundamental type, namely, a heart-shaped twin 
haying an angle of 60° at the summit. These crystals are formed 
of minute prisms that adhere to one another very perfectly, of 
which I have spoken above. ‘These sections are covered with 
parallel striz on both sides of the triangles, producing very dis- 
