Vol. 6, 1920 
GEOLOGY: N. L. BO WEN 
161 
It seems possible that even a dike-like mass of basaltic magma if de- 
formed at an early stage of crystallization, might give rise, in the por- 
tions that are constricted by deformation to a peridotitic mass, though 
probably not as readily as in the case of a sheet. The mutual approach 
of the walls necessitates a movement of liquid out of the intervening space 
but close to the walls there would be a certain amount of drag of the crys- 
tals resulting in some movement of liquid relative to crystals. The two 
marginal regions of concentration of crystals might be brought together 
and even crystal packing and the expressing of liquid might occur if the 
constriction were carried far enough. Thus might result a dike which, 
intersected at this particular point, would be simply a peridotite. It 
would, therefore, seem necessary to include the above action among those 
to be considered as capable of producing dike ''intrusions" of peridotite. 
Complementary dikes. — Those minor but rather constant associates of 
intrusive masses, the complementary dikes, aplitic and lamprophyric, 
may perhaps be produced by action of the same general nature as that we 
are now considering. In very late stages of crystallization residual liquid 
may be drained into clefts in the mass and there suffer differentiation as 
a result of local pinching and swelling during crystallization. This is 
particularly likely to have important effects in such small bodies. Comple- 
mentary units would be formed that might readily have a composition 
not matched in large intrusive masses. 
Primary banding. — A further effect of deformation during crystalliza- 
tion appears to be that which finds its expression in primary banding. 
Shearing of a crystalline mesh, particularly when it is still weakly knit 
together and permits passage of liquid through it with great freedom,^ 
may develop a feature of the same general nature as a shear zone developed 
in solid rocks. Lenticular openings may be formed that instantly fill 
with liquid from the interstices of the mesh and repetition of the action 
may give rise to a banding, properly oriented with respect to the shearing 
stress, and showing approximately that contrast between bands that 
obtained between liquid and crystals at the time of the action. The crys- 
tals that became detached during the shearing would naturally become 
aligned in the liquid filling the lenses so that fluxion structure would be a 
natural accompaniment. Moreover, in the larger lenses developed a 
gravitative sorting of these detached crystals would take place under par- 
ticularly favorable conditions and bands showing the extreme contrast 
of monomineralic types might thus be formed. The banded gabbros ex- 
hibit features that appear to match the results of the postulated action. 
Other rock types should be subject to similar action, but are probably 
not as likely to give such obvious contrasts in the bands as are the gabbros. 
Alkaline rocks. — In rare instances rocks are found that have empty 
interstices and to these the term miarolitic is applied. It is usually as- 
sumed in explanation that the rock had crystallized in such a way as to 
