VEINLETS IN THE SILURIAN AND DEVONIAN 78 
According to the author’s theory, the fibrous veins owe their 
peculiar structure to the fact that the material for growth was sup- 
plied only to the base of the growing crystals through solutions 
occupying closely spaced capillary or subcapillary openings in the 
walls, while the non-fibrous veins were deposited from solutions 
that entered between the walls of narrow capillary fractures and 
bedding planes. Because of the slow rate of circulation through 
such minute spaces, diffusion through the solution is probably an 
important factor in supplying material to the growing veins. 
The diameter of the calcite and gypsum fibers varies with the 
spacing of the openings through which the material for their growth 
is supplied, and is independent of the size of the veins. There is 
evidence of some recrystallization within the non-fibrous veins 
during the process of growth, as a result of which those crystals 
that for any reason are less stable than their neighbors are redis- 
solved, thus furnishing additional material for the growth of others. 
The field evidence, confirmed by laboratory experiments, indi- 
cates that the veins were not deposited in pre-existing openings, 
but that the growing veins have made room for themselves by push- 
ing apart the inclosing walls. The presence of drusy cavities, 
banding, or crustification are not in themselves proof that a vein 
was deposited in a pre-existing open fissure. 
The force that enables a growing vein to make room for itself 
is attributed chiefly to the molecular forces associated with the 
separation of solids from solution. 
