64 STEPHEN TABER 
Most of the objections raised against these theories of vein forma- 
tion are equally applicable in the case of the veins of fibrous calcite 
and gypsum; and, in the descriptions given above, much confirma- 
tory evidence may be found. All of the structural features char- _ 
acteristic of these veins have been duplicated in fibrous veins grown 
in the laboratory where their origin and growth could be observed 
in detail.t ‘In view of all the facts obtained from field investiga- 
tions and laboratory experiments, the conclusion is inevitable that 
the veins of fibrous calcite and gypsum have been formed through 
a process of lateral secretion, the growing veins making room for 
themselves by pushing apart the inclosing walls, and that the 
fibrous structure is due to the circumstance that the material for 
crystal growth was accessible in only one direction. 
Calcite and gypsum are not normally fibrous, and wherever they 
have developed this structure it is due to the physical conditions 
which have prevented crystal growth, except in one direction. 
Merrill has described fibrous incrustations of gypsum forming on the 
walls of caves, and notes that the growing crystals not infrequently 
force off pieces of the limestone of considerable size.” 
Laboratory experiments and field investigations indicate that 
the essential conditions for the growth of fibrous minerals, such as 
calcite and gypsum, are: (1) the growing crystals must be in 
contact at their base with a supersaturated solution; and (2) the so- 
lution must be supplied through closely spaced capillary or subcapil- 
lary openings in the surface of the wall rock. In the fine-grained 
limestones and shales the constituent particles are relatively small, 
and therefore the open spaces which are chiefly subcapillary in size 
are closely spaced; but in the crystalline ‘‘gray limestone” with 
its coarser texture these openings while no larger are necessarily 
more widely spaced. This explains the coarse texture of the fibrous 
veins occurring in the “gray limestone.” The coarsely crystalline 
non-fibrous structure of veins where they pass through chert masses 
is due to the relative impermeability of the chert which has here 
«Taber, ‘“The Origin of Veins of the Asbestiform Minerals,” Proc. Nat. Acad. 
Sci., II (1916), 659-64; and ““The Genesis of Asbestos and Asbestiform Minerals,” 
Bull. Am. Inst. Min. Eng. No. 119, 1916, pp. 1973-08. 
2G. P. Merrill, ““On the Formation of Stalactites and Gypsum Incrustations in 
Caves,” Proc. U.S. Nat. Mus., XVII (1894), 81. 
