278 Transactions of the Royal Microscopical Society. 
tinctly those striae which erystallographers generally call osec/- 
latory. 
it is not the microliths of Sart, however, that show the most 
remarkable crystalline forms. Some specimens of the whetstone of 
Ottrez exhibit, with a microscope of 600 or 700 diameters magni- 
fying power, a multitude of minute triangular crystals of the 
utmost delicacy and perfection (Fig. ¢). It has been shown that these 
little geometric solids are to be classed, on account of their angular 
measure and their form, with the geometrically disposed microliths 
of Sart and with the geniculated twins. But, on the other hand, 
inasmuch as the fundamental character seems to point them out as 
belonging to those regular aggregations, even in so much do they 
differ from them by their perfect transparency. They are not 
formed by the gathering together of a number of minute prisms, as 
is the case in the specimens from Sart, and are perfectly colourless. 
‘They are exceedingly small, the base often not measuring more 
than the thousandth of a millimeter. That some idea may be had 
of their delicacy, it may suffice to state that it is not rare to find 
two or three of them superposed. If the observer turn the micro- 
metric screw he can readily convince himself that they occupy 
different planes even in the extremely minute thickness of the thin 
sections. Thanks to the delicacy and the perfection of their forms, 
one may attempt, with some chance of success, to determine the 
crystalline type in which the mineral is to be classed. The 
minuteness, however, of its dimensions, and the impossibility of 
isolating it, impose the necessity of pronouncing with great reserve 
‘on the mineralogical nature of these remarkable microscopic twins 
to which I now for the first time call attention. 
By the inspection of the outline and of the extremely delicate 
line that joins the summit with the obtuse angle opposed the ob- 
server may readily recognize hemitropical forms; and the two 
halves polarizing with complementary colours prove in their turn 
that these two parts have their optical axes placed as they should 
be in the case of a hemitropy similar to that which we find in the 
ereater number of these twins. 
This mineral constitutes, in the great majority of cases, twins by 
juxtaposition ; however, some are found which are twins by pene- 
tration ; those, for example, which cling together by their summits, 
recalling what one sees in certain twins of tridymite, mentioned by 
Vom Rath. The angles of these rhomboids, to judge them by 
approximate valuations, such as can be made by the microscope, are 
of 60°, 90°, and 120°. In other terms, these are the angles given 
by the hemitropies of the crystals of the rhombic system which 
have an edge of about 120°, and for which the hemitropy is made 
following the rule = Plane of hemitropy = a face of the prism of 
about 120°, that is to say, a dome, 3 P oo for example, the principal 
axis of the two individuals forming together an angle of about 60°. 
