GEOLOGY: S. TABER 
y and for the most part they are limited to metamorphic rocks. The 
primary minerals of igneous rocks are never fibrous. Many minerals 
without prismatic cleavage have fibrous varieties. They differ from as- 
bestos in having fibers that are usually coarser, more brittle and not 
so easily separable. With fibers of equal size the flexibility and tensile 
strength are probably determined chiefly by chemical composition. When 
minerals do not have a columnar or prismatic habit, the fibrous struc- 
ture is obviously due to mechanical conditions which have prevented 
growth except in one direction. 
The shape of a growing crystal is controlled by one or more of three 
independent factors, namely: (1) the tendency to assume a regular 
polyhedral form because of the forces of surface tension and molecular 
orientation; (2) the relative and absolute magnitude of the external 
forces resisting growth in different directions; and (3) the accessibihty 
of the material from which the crystal is built. As a result of the first 
factor some substances normally crystallize in slender acicular or hair- 
like forms, but an asbestiform structure is never produced without the 
assistance of one of the other two factors. Crystal growth under un- 
equal pressure undoubtedly explains the development of fibrous struc- 
ture in some instances. The solubility of most substances increases 
with the pressure and therefore crystal growth may be limited to the 
direction of least pressure. This hypothesis is not applicable, how- 
ever, to fibers that develop normal to the walls of a vein. Moreover 
the veins frequently intersect and run in all directions through a given 
rock mass. Such occurrences of abnormal fibrous structure must there- 
fore be attributed largely to the circumstance that the material for 
growth was accessible only in one direction. 
The efficacy of this method of producing fibrous structure in crystals 
has been proved experimentally. ^ Cups of porous porcelain were part- 
ly immersed in concentrated solutions of copper sulphate, alum, and 
other salts, thus permitting the solutions to be drawn by capillary 
attraction into the upper and exposed walls of the cups. When the 
solutions occupying the small pores were supersaturated by evapo- 
ration or cooling so that crystallization began, fibrous crystals grew slow- 
ly outward from the surfaces of the cups. 
Crystals also developed at favorable places within the walls of the 
cups, which were ruptured by the pressure as growth continued. The 
fractures thus formed were gradually extended and widened by the 
growth of fibrous veins, closely resembhng in structure the cross-fiber 
veins found in rocks. The crystals have a fibrous structure because 
additions of new material were made only at their base. Using alum 
