280 Transactions of the Royal Microscopical Society. 
us for the oligistiferous phyllades. Here we have as guide 
M. Zirkel, who examined some thin sections of the phyllade of 
Recht, which is definitively the same as that associated with the 
Belgian whetstones. My observations on the oligistiferous phyllade 
of Ottrez, Bihain, Viel-Salm, &c., agree in every point with those 
of M. Zirkel. 
. M. Zirkel finds that the red grains scattered throughout this 
rock are in fact, as Dumont had admitted, oligiste iron; that under 
the microscope they appear of a red colour, and the sections, though 
generally irregular, are, however, sometimes hexagonal. M. Zirkel 
attributes the red colour of the phyllade to the accumulation of 
lamellz of this mineral. These lamella, as well as the other con- 
stituents of this rock, are enclosed in a micaceous substance, which 
constitutes the fundamental mass of the schist. The third con- 
stituent recognized by M. Zirkel is garnet, which appears in his 
preparations with the same characteristic marks that we have 
already seen in the whetstone. We would remark, however, that 
this mineral is far more abundant in this rock than in the slate. 
He makes mention besides of prismatic microscopical crystals, some 
of which are geniculated twins, and a fifth mmeral composed of 
granules generally flattened, black and opaque, irregularly termi- 
nated, and which are less than 0°'015 mm. He is inelined to 
regard them as carbonaceous particles so often found in black or 
blue schist. . 
We have but little to add to this excellent description of oligis- 
tiferous slate; the only additional element that we have yet recog- 
nized in plates of this rock is schorl, whose sections appear here 
like the sections of this mineral which we have examined in 
describing the whetstone of Salm. 
If now we compare the results which M. Zirkel has obtained for 
the composition of the oligistiferous slate and those which we have 
noted in this communication, we shall remark striking analogies in 
these two rocks, which we were far from suspecting, but which per- 
fectly explain the phenomena presented by their microscopic study. 
In both cases a micaceous substance constitutes the fundamental 
mass; there is the same structure, both contain garnet and small 
prisms quite identical, and in both schorl is present. The difference 
consists only in the fact that the slate contains lamelle of oligiste 
iron and carbonaceous granules, while these two minerals are rare 
in the whetstone. 
There yet remains an important geological question to be dis- 
cussed: it has reference to the origin itself of the rocks. Let it 
suffice to renrark that I have not met there any elements with 
traces of clasticity. On the contrary, everything seems to show 
that its essential elements are crystalline, formed cn situ. Resting 
on the ensemble of facts which I have observed, stratigraphic as 
