240 J. D. TRUEMAN 
saturation and differential pressure appear to be the most important 
of these conditions. In either of these cases the resulting form of 
the mineral seems to depend to a marked degree upon the char- 
acter of its cleavage. The elongation of quartz, on the other 
hand, a mineral which possesses no good cleavage, is, under differ- 
ential pressure at least, not marked and is apparently independent 
of crystallographic directions. The only possible exception noted to 
these generalizations is feldspar. While feldspars crystallized from 
viscous melts are, apparently, elongated parallel to the two principal 
cleavages, in the grains formed under differential pressure there does 
not appear to be any relation between the direction of elongation and 
the position of mineral cleavage. This is possibly to be explained 
by the number of cleavage planes in that mineral or to the position 
of the two principal cleavages at right angles to each other. 
Pirsson* has explained the development of mineral elongation in 
viscous melts by differences in molecular attraction which are con- 
sidered to exist between directions within and across the cleavage. 
Since the same forces act during the normal crystallization of miner- 
als, this might lead one to think that the elongation of crystals due 
to the conditions of the solution or to differential pressure repre- 
sented only a more pronounced development of differences in 
dimensions existing in crystals of normal development. That 
this is not so, and that the elongation of minerals which have 
developed under favorable conditions may be independent of 
mineral cleavage is shown by the well-known prismatic crystals 
of quartz, found so frequently in cavities, notwithstanding the 
fact that quartz is a mineral which possesses no good cleavage. 
It seems a more general rule that uniaxial minerals, when freely 
developed, are elongated in the direction of the vertical axis, no 
matter what the direction of mineral cleavage. Apatite, beryl, 
and corundum, for example, appear to be all normally elongated at 
right angles to their best plane of cleavage. The same is true for 
the orthorhombic minerals topaz and danburite. Among rocks, 
pegmatites probably offer the most favorable conditions for the 
development of biotite crystals and here they are frequently, if not 
generally, elongated at right angles to their cleavage. 
t Am. Jour. Sci., XXX (1910), 110. 
