OPTIC AXES IN HARMOTOME AND WOHLERITE. 
567 
The figures 1, 2, and 3 belong to the simple macles which constitute the Morvenite of 
Scotland and the greater part of the crystals of Oberstein ; the figures 4 and 5 are the 
double cross-formed macles so common at Andreasberg. In the first the two individuals 
interpenetrate one into the other so as to form four sectors, usually very unequal in their 
magnitude. Theoretically these sectors ought to occur following two planes perpen- 
dicular to each other, the one parallel to the base, the other parallel to the very simple 
modification a}, the inclination of which upon this base is precisely 90° ; but in reality 
these sectors more frequently are united only in an imperfect manner, and there exist 
between their sides little channels more or less sinuous (fig. 6). It is easy to show this 
by passing parallel rays of polarized light through plates cut very thin parallel to the 
plane of symmetry. 
In the cross-formed macles (figs. 4 and 5) two individuals, already composite, penetrate 
one another so as to have their planes of symmetry perpendicular, but their intercrossing, 
instead of taking place on the planes inclined at 135° on the base, and very nearly in the 
plane e\ usually occurs with surfaces that are irregular and jagged : however, the direc- 
tion of what is theoretically the plane of assemblage is sometimes indicated by a series 
of plates excessively thin and which fill the little interval, then sensibly rectilinear, which 
exists between the two individuals (fig. 7). 
It is well known that on crystals of TIarmotome it is very difficult to obtain measure- 
ments for the angles that are at all precise ; nor can it be otherwise ; for independently 
of the crossed strife the points of junction of which form upon the plane of sym- 
metry (f two slightly salient lines perpendicular to each other, the base p is almost 
always more or less decidedly waved with lines parallel to its intersection with h\ and 
the faces to are finely striated in the direction of the edge jx ; furthermore, the indivi- 
duals of which the macles are composed are hardly ever united in a manner suffi- 
ciently perfect for the four faces h l and ,?/ of a crystal similar to that represented in 
fig. 1, to be found exactly in the same zone with the two basest? ; the same is true of 
the faces anterior and posterior to A 1 to, which ought to form one single vertical zone. 
Thus it is only an exceptional case, or where one is operating on very small crystals of 
Morvenite from Strontian, that one can obtain the inclinations p to and A 1 to with any 
exactitude. 
Combining the results formerly known with those which a new and very numerous 
series of observations have afforded me, the following Table will give the inclinations at 
which I have now arrived. 
5: A :: 1000:1007,00 D=818,02 d=575,19. 
Plane angle of the base . . . =109° 46' 27". 
Plane angle of the lateral faces . =109° 10' 50". 
4 i 
MDCCCLXVIII. 
