236 I. D  ERGE MAN: 
minerals and the conditions present during their formation rather 
than the relation of the minerals to their order of crystallization. 
According to the views previously outlined it should be, and 
actually is, a characteristic of metamorphic rocks rather than 
normal igneous varieties. As many minerals present in primary 
_ gneisses seem to possess an abnormal elongated habit, it is hoped 
that a discussion of this feature may throw some light on the 
texture and mode of origin of these rocks. 
Elongation of minerals under viscous conditions in an igneous 
melt.—Pirsson' has recently published an interesting article in 
which the effect of viscosity on mineral habit is discussed. His 
observations on sections of rocks which have solidified quickly 
show that the minerals of such rocks tend to assume tabular or 
needle-like forms. It was especially noted that the elongation is 
generally parallel to prominent cleavage faces. Pirsson attributes 
this to the fact that the cohesive attraction within the plane of 
cleavage is greater than that across it. He supposes that viscosity 
may become so great that the molecular attraction across the 
mineral cleavage is insufficient to orient additional material so 
that the mineral becomes elongated in the direction of its cleavage. 
The growth of the crystal end, he considers, may be aided by the 
mobility imparted to the surrounding liquid by the heat of crystal- 
lization. Miers? has explained somewhat similar cases of elongation 
of crystals by the supersaturation of the surrounding solution. 
He considers that the end of the crystal may be able to remain 
continually in strongly supersaturated solution and, accordingly, 
grows more rapidly in one direction. ‘This seems to be a somewhat 
different explanation from that given by Pirsson. While opinions 
regarding the cause of mineral elongation may vary, the fact that 
minerals do assume elongated habits when growing under viscous 
conditions in a magma and that the elongation takes place very 
frequently parallel to prominent cleavage directions seems to. be 
pretty well established. 
Elongation of minerals due to differential pressure-—That rocks 
which have recrystallized under great pressure are characterized 
'L. V. Pirsson, ‘On an Artificial Lava-Flow and its Spherulitic Crystallization,” 
Am. Jour. Sci., XXX (1910), 97. 
2H. A. Miers, Science Progress, II (1907), 128-20. 
