216 W. M. Hutchings—Ottrelite in North Cornwall. 
mineral has undergone during rock-movements. These cleavages 
are best studied on flakes isolated from the rock and separately 
mounted. The two most important of these secondary cleavages 
intersect on the basal plane at an angle of 120° or an approxima- 
_ tion to it. Many measurements give that angle exactly, while many 
more vary but little from it, though some diverge more markedly. 
Rosenbusch states that there is a third cleavage, less well developed, 
bisecting the obtuse angle of the other two. Cracks answering to 
this description may be seen in some flakes of the Cornish mineral, 
but they are few and not very marked. The French writers speak 
of a cleavage approximately vertical to one of those intersecting at 
120°, and many cracks corresponding to this may also be seen in 
the mineral now under consideration. But it really does not seem 
possible to say that either is really a definite cleavage, or to be in 
any way certain concerning any third line of cleavage observable on 
the basal plane. The cleavage-cracks are nearly all quite vertical to 
the basal plane, and are seen on the lath-shaped cross-sections of the 
flakes as lines nearly always at right angles to the lateral boundaries, 
obliquity being rare and very slight. 
Most of the ottrelite flakes are intergrown with crystals of white 
mica, so that nearly all the narrow lath-shaped sections are seen to 
be united with one, or usually with two, flakes of the mica, one on 
each side, the individuals lying perfectly parallel, and in cases where 
the ottrelite is so cut that it extinguishes parallel to its length, the 
whole combination extinguishes together. The petrologists who 
have described the rocks of the Ardennes do not appear to have 
observed a similar intergrowth of ottrelite and mica. 
Gosselet considers (Htudes sur l’Origine de l’Ottrélite, Annales de 
la Société géologique du Nord, tome xv. p. 85) that in the rocks 
he studied the ottrelite was formed at a time after the phyllites were 
already fully developed as such. This appears to be equally the 
case in these Cornish slates, and it is quite evident on observation 
of structure and arrangement, that the white mica with which the 
ottrelite is intergrown was formed at the same time. It is in larger 
and much more definitely-bounded individuals than the main sericite 
of the rock. There are also crystals, apparently of the same genera- 
tion, but not intergrown with ottrelite. The dimensions of some of 
these average about 725 inch in length by half that width, in sections 
vertical to the base. 
This intergrowth with mica renders much of the optic examination 
of the ottrelite impossible in the slate direct. By pulverizing some of 
the rock moderately fine, a number of the thicker flakes may easily be 
detached from the mica, and are then easily separated out by means 
of a Sonstadt solution of sp.g. of about 3, most of the thinner flakes 
remaining suspended attached to mica. Material so separated is 
suited for optic examination, and, as above stated, for study of the 
cleavages, much of it being in sharply-bounded cleavage-fragments. 
In colour the Cornish mineral is more inclined to blue than the 
ottrelite of Ottrez. The characteristic pleochroism is strongly 
developed. The scheme is a=yellowish green, B=blue, y=pale 
greenish yellow. 
