276 Transactions of the Royal Microscopical Society. 
ordinarily curved, a fact we have remarked ourselves. As we have 
said, it is not rare to find the sections of schorl furrowed by fissures 
more or less parallel to the base. It was believed by some micro- 
scopists that this fissure represented a cleavage. Now, the best 
mineralogical works make no mention of a cleavage in this direction. 
Dana, Naumanm, and Des Cloizeaux mention cleavages only follow- 
ing R, o P2; besides this, these cleavages are not easy. We are 
therefore inclined to admit that these ruptures are the effect of 
mechanical action, as, for example, the stretching of the beds at the 
moment of foliation. The rupture in these little prisms must neces- 
sarily be made parallel to the base, where the points of more feeble 
resistance are found. I add that M. Zirkel and myself have been 
able to prove that these prisms belong to the hexagonal system. 
Tn a research that I made with him, we have found that in a 
thin section of a schist of the Ardennes a hexagonal section of this 
dicroscopic mineral was dark between the crossed Nicols’ prisms. 
This fact wholly confirms our interpretation as to the crystalline 
system of these little prisms. 
In the few pages devoted by M. Zirkel to a microscopic de- 
scription of the slates of Recht, he pointed out the presence of a 
prismatic mineral of a greenish yellow colour. High powers of 
the microscope are needed to observe this mineral, the prisms not 
being more than 0°03 mm. in length and 0°005 mm. in thickness, 
so that it is difficult to determine all the faces of the crystals, which, 
though complete, have not very well defined edges. Irregular 
aggregations are often found composed, sometimes of one or two 
individuals, sometimes of twins in the form of a knee. As none of 
the characteristics of these small crystals are opposed to those of 
augite,* M. Zirkel believed that he could class it with that mineral. 
Such are, in brief, the micrographic details he gives upon these 
microliths. 
We find them again in great abundance in the whetstone, and 
there under a great variety of very interesting crystalline forms, 
which have not as yet been pointed out by any micrographer. 
These prisms, which are identical with those remarked by M. Zirkel, 
are scattered through the whetstone sporadically, and are dis- 
tinguished from the prisms of schorl by their form, by the way 
they are grouped, by their tint, and also by their exiguity. At 
different times these prisms are ranged in lines, verge towards each 
other, and interlace, maintaining the while almost constant angles 
in their superposition. In some sections it is remarked that the 
prisms follow the undulations of the micaceous substance. We 
shall add nothing to the excellent description which M. Zirkel has 
* On this subject M. Zirkel remarks that our friend C. A. Lossen, of 
Berlin, has found in inferior Devonian, near Winterburg, some crystalline schists 
in which the macroscopic augite appears as an essential element. 
