- 
i. 
Bartow—A Mechanical Cause of Homogeneity of Crystals. 639 
_ Etched figures on the crystal faces appear to have a form 
quite independent of the twinning which takes place, showing 
that the latter does not effect sufficient change of structure to 
modify this phenomenon.’ 
Probably the optical anomalies presented by fluorspar have 
a somewhat analogous origin. This suggestion receives support 
from the fact that sections taken from the middle of a crystal 
showed the same optical behaviour as those taken near the 
surface, and it is in harmony with the fact that the optical peculia- 
rities remain practically unchanged by rise in temperature,” which 
they would not be likely to do if the optical anomalies were 
traceable to strain. Brauns notes that the modifications in the 
etched figures observed in a specimen of fluorspar from Cornwall 
are of such a nature as to be explicable by the assumption that 
linear disturbances of the molecular structure having a direction 
parallel to the axis of the accretion segment (Anwachskegel) are 
present which enable the solvent employed to penetrate more readily 
in this than in the other direction.* 
Brauns regards microcline as of secondary origin, and it is 
interesting to observe that a slight shearing of comparatively thin 
layers, due to a dimorphous change acting on an intractible mass, 
7. €., 00 & mass in which general change of shape is prevented, 
would be competent, as in the case of leucite above referred to, to 
produce lamination.* 
And in connection with this it is interesting to note that 
Forstner has succeeded-in making a potash felspar containing 
sodium pass to the monosymmetric form by heating it.° 
Microclines, being mixed crystals, some light may be. thrown 
upon the effect of their composition on their stability and readi- 
ness to undergo a dimorphous change, when we come later on to 
consider the probable structure of mixed crystals generally.* 
1 Brauns, Joc. cit., p. 116 ; and see Erb in Neues Jahrbuch VI., Beilage Band, pp. 
121-150, 1889. 
2 Braung, Joc. cit., p. 3384, &e. 3 Ibid., p. 337. 
*Comp. Jdid., p. 133, &e. Brauns’ theory is that microcline was originally 
monoclinic, but, owing to the presence of soda is endowed with instability of form, 
and thus that long-continued pressure due to gradual mountain movements has been 
enabled to alter it to the triclinic symmetry. Jbid., p. 146. 
5 Ibid., p. 189. 8 See p. 647. Comp. Brauns, Joc. cit., p. 139. 
SCIEN. PROC. R.D.S., VOL. VIII., PART VI. 3A 
