1885.] Mineralogy and Petrography. 297 
cal form of a higher grade of symmetry than they themselves 
actually possess, by complicated and repeated twinning. Klein, 
Krocke and certain other German mineralogists, on the contrary, 
maintained that the optical peculiarities were produced, not by 
twinning but by a molecular disturbance due to an internal ten- 
sion caused by the irregular growth of the crystal} Klein had 
even succeeded in proving that the apparent twinning lines—the 
boundary between the areas of different optical orientation— 
could be made to move by raising the temperature,’ and the inter- 
pretation put by the same writer on the interesting observation 
of Mallard is very important, as offering a possible means of 
reconciling the different views which have hitherto existed re- 
garding these anomalous optical phenomena. Klein suggests 
that the fact that boracite becomes isotropic above 265° C., proves 
that this substance is dimorphous, crystallizing in the regular 
system above, and in some other, probably the orthorhombic, be- 
low this temperature’ The crystals must therefore have been 
formed at a temperature above 265° C., and hence possess geo- 
metrically the regular shape; their internal condition is due to 
the effort on the part of the molecules to rearrange themselves 
in accordance with the altered conditions, which is strong enough 
to produce a profound optical disturbance, but is not able to pull 
to pieces the crystalline form. Boracite crystals at ordinary tem- 
peratures are, in fact, a kind of paramorph, composed of optically 
orthorhombic portions enclosed in the regular framework in 
which the substance originally, at a higher temperature, crys- 
tallized. 
Still more recently Klein* has shown that leucite becomes iso- 
tropic at high temperatures, and Merian’ obtained a similar result 
for tridymite ; thus it seems very probable that many other sub- 
stances, like garnet, analcite, perofskite, senarmontite, etc., which 
show optical properties in no accord with their external form may 
be satisfactorily explained by the assumption that they are dimor- 
phous. _ 
Another class of substances like potassium npea leadhillite, 
aragonite, etc., whose optical properties at ordinary temperatures 
ite tn perfect accord with their crystalline form, have been shown 
capable of transference by increase of temperature to a state 
where their optical properties have a higher grade of symmetry, 
and consequently are no longer what the form would require, ¢. g., 
1 Neues Jahrbuch fiir Min., etc., 1881, 1, pp. 255 and 256. 
216. p. 239, et seq. 
3 Neues Jahrbuch fiir Min., etc., 1884, I, p. 239. 74. ref. pp. 185 and 186. 
4 Nachrichten d. Kan. Ges. d. Wissensch. zu Göttingen, May, 1884, and Neues 
Jahrbuch fiir Min., etc., 1884, II, p. 50. 
5 Neues Jahrbuch für Min., etc., 1884, I, p. 193- 
ê Mallard: Bull. Soc. Min. de France, T. v, 1882, p. 219. 
