THE ORIGIN OF FOLIATED CRYSTALLINE ROCKS 235 
have the smallest molecular volume or the highest specific gravity. 
As an explanation, also, for the unusual development of cleavage 
faces in certain minerals, he suggests that the molecular arrange- 
ment in such minerals is denser within the plane of mineral cleavage 
than across it. Though the suggestion of parallelism between 
perfection of crystal form and molecular density is interesting and 
probably significant, it must not be forgotten that the characteris- 
tics of crystalloblastic texture previously enumerated are based 
almost solely on observation. 
Recent opinions on the crystallization of igneous rocks.—Several 
writers have recently questioned the views commonly held regard- 
ing the order of crystallization of igneous rocks. It has been sug- 
gested," for example, that in the important class of the diabases 
the crystallization of the different minerals has been approximately 
simultaneous. In such cases it might be supposed that the relative 
development of crystal form was not entirely dependent upon the 
order of crystallization and that possibly the resulting texture 
might be confused with the metamorphic type. Thus in graphic 
granite,? where the outlines of the quartz areas agree more or less 
with the cleavage directions of the feldspars, the relations ot the 
two minerals would seem to be dependent upon the individual 
properties of the minerals rather than upon their order of crystalliza- 
tion. Notwithstanding such cases of apparent simultaneous crys- 
tallization, it will probably be generally agreed that there is no reason 
yet known for thinking that order of crystallization is not the most 
important factor in determining the mineral outlines of normal 
igneous rocks. Grubenmann has pointed out, moreover, that 
there are important differences between the texture produced 
by simultaneous crystallization from an igneous melt and that 
resulting from recrystallization. 
THE SIGNIFICANCE OF THE ELONGATED HABIT ASSUMED BY 
MINERALS WHEN GROWING UNDER CERTAIN CONDITIONS 
The development of columnar or platy habit in minerals of a 
rock is a feature which is controlled by the individuality of the 
tC. N. Fenner, ‘‘The Crystallization of a Basaltic Magma from the Standpoint of 
Physical Chemistry,” Am. Jour. Sci., XXIX (1910), 220. 
2L. V. Pirsson, ‘“‘Rocks and Rock Minerals” (1908), 212. 
