Economic Geological Deposits, — Crosby. ^."V.) 
cordance with the mode of origin of the cavities, it is of the 
first importance to distinguish sharply, as Posepny lias done, 
between the cavities formed b} T mechanical forces (spaces of 
discission) and those formed by chemical forces (spaces of 
dissolution). Of the eight types of veins described in the 
following pages, six belong to the first category and two to 
the second. 
(a) True fissure veins, filling profound fissures, which art' usually ac- 
companied by displacemfenl (fault fissures). This is the Leading type of 
veins and is so recognized in the standard text-books of geology. When 
the fissures can be proved to coincide with important displacements, 
permanence in depth is reasonably assured and filling by ascending cur- 
rents may be regarded as highly probable. Slickensided walls and a 
distinct selvage are very characteristic features, though by no means 
essential or peculiar. Important examples are so numerous as to make 
citation or further description unnecessary. 
(b) Crushed-zone or shear veins, filling crushed or shear /.ones along 
faults or lines of shearing stress. These are often equivalent to fissure 
veins filled with fragments of the wall-rock: and in other cases the 
crushing may take the form of a more or less distinct sheeting. The 
•crushed or sheeted rock is frequently extensively impregnated or re- 
placed by the vein matter: and the enclosure of the fragments is often 
so dike-like that the vein matter has. even by some recent writers, been 
regarded as of igneous origin. The veins of this type are commonly 
without definite boundaries or walls, and sometimes pass into more or 
less typical stockworks. The normal form of the deposit, when not 
sheeted, is believed to be approximately or roughly lenticular; and it is 
probable that one greal lens may succeed another in strike or depth 
along an extended line of fracture. This localization of the crushing 
may, apparently, be regarded as a necessary result of a shearing stress 
along a highly irregular or somewhat interlocking fracl un\ t he protrud- 
ing or convex portions only of eit her wall being crushed by t he enormous 
friction. Among the more importanl deposits believed to be referable 
to this type are the nickel ores of the Sudbury district. Ontario, and 
the (Jap mine, Pennsylvania. 
(c) Joint or gash veins, filling joint-cracks, or fissures due to (]) 
shrinkage and (2) vibratory and torsional strains, and C-U transverse ris 
sures due to plication and warping without faulting. This type, with 
varying definition, lias been recognized in several classifications. of veins. 
It is. of course, not always sharph distinguished from the true fissure 
veins. The chief points of difference are. th.it faulting is more com- 
monly absent and when occurring is less extensive, that the veins are 
more limited in length and depth, that slickensided walls and a true 
selvage are more commonlj wanting, and the filling is more likelj to 
have been by lateral secretion. Like joint-cracks, the gash veins are 
frequently intersecting (reticulated veins) and often exhibit the Btock- 
