270 Transactions of the Royal Microscopical Society. 
material, and another of bluish slate distinctly separated by a 
straight line, sometimes of perfect regularity, there is a tendency 
to suppose that they are two fragments of different rocks fastened 
together one above the other, the juxtaposition being due rather to 
art than to nature; but when these stones are seen in the place 
which they occupy in the strata, one readily observes that the vein 
of whetstone is intimately united to the slate, and that the work- 
man has nothing else to do than to square the fragments. In 
certain exceptional cases, however, art is made to imitate nature ; 
and the workman sometimes, by fastening together fragments of 
different colours, gives to the market a hone that differs not in 
appearance from those formed by nature. 
Although this line of demarcation is usually found very distinct 
between the two layers and marking the direction of stratification, 
it also happens frequently that the whetstone passes imperceptibly 
into the slate, in such a way that at a certain point it is no longer 
possible to distinguish whether slate or whetstone is in question. 
It might be said there was a sort of mutual penetration of the two 
layers. 
: There is one character, common to the veins and the incasing 
layers, to which special attention should be given. It is this: the 
foliation, oblique to the stratification, is prolonged from the slate 
into the whetstone. Sometimes this foliation is but faintly charac- 
terized in the compact varieties, but it is always present in a latent 
state, and if the slate be broken, the lamina comes off, passing across 
the whetstone, and the fissure is prolonged with a regularity and 
constancy of angle which shows evidently that the two rocks have 
a common cleavage. I have proved that there is also present in 
these two rocks a second cleavage, not so easy, but, like the first, 
clearly distinct from the stratification. The existence of this 
cleavage shows, first, that the layers of whetstone existed before 
the uplifting of the strata; and the coexistence of the two 
cleavages obliges us to admit that the rocks of this group have 
been subjected on two different occasions to a pressure, brought to 
bear in two different directions, as contended in similar cases by 
Mr. Sorby. The easier cleavage must have been produced while 
the rocks were still in the plastic state, the second when they were 
already more solidified. I have dwelt upon the characters common 
to the two rocks, because they are in close relationship with the 
details of microscopical analysis which follow. To sum up, the 
whetstones are contemporary layers with the incasing slates, and 
have been subjected to the same mechanical actions as all of the 
phylladic rocks of this group. 
After this exposition, belonging rather to geology, let us inquire 
what are the minerals that constitute the whetstone of the neigh- 
bourhood of Salm. It should be remarked that this question cannot 
a 
