


ORE-FORMATIONS 259 
in the form of water-rolled fragments, associated with other 
sedimentary materials. It may be inferred, therefore, that 
these have probably been derived from some pre-existing 
formation, which has been broken up at the surface and the 
débris introduced underground by the mechanical action of 
water. 
Irregularly shaped masses of hematite occurring in lime- 
stone may sometimes have been deposited in caverns and 
underground water-courses, and a similar origin has been 
assigned to many analogous masses of galena and zinc-blende 
enclosed in the limestones of various regions. The joints 
and even the bedding-planes of a limestone, in the vicinity of 

FIG. 104.—VEINS IN LIMESTONE. 
1, limestone; sh, shales; 6, b, bunches; f, flat; g, g, gash-veins. 
a “mass,” are frequently charged with the same ore, forming 
what are known as flats, gash-veins, pockets, bunches, pipes, 
nests, etc. (Fig. 104). It is very doubtful, however, whether 
the ore-masses in question occupy pre-existing cavities. 
Probably most of them should be included in the next 
group (0). 
(0) Masses due to Metasomatic Replacement.—This remark- 
able change is well illustrated by the transformations 
undergone by limestone, which is sometimes replaced by 
ores of iron, lead, zinc, or silver. Some of the masses of red 
hematite met with in the Carboniferous limestone of 
Cumberland, are clearly cases of metasomatic replacement, 
and possibly, as already suggested, the same is true of them 

