

258 STRUCTURAL AND FIELD GEOLOGY 
be doubted—not infrequently perhaps effected during the 
period of metamorphism, while others would seem to have 
been introduced at a later date. 
3. IRREGULAR ORE-FORMATIONS 
1. Masses.— The ore-formations grouped under this head 
are met with chiefly in limestones. Sometimes they occupy 
underground cavities—the deserted courses of subterranean 
waters—which they partially or completely fill; in other cases 
the ore-formation would appear to be the result of meta- 
somatic replacement—that is to say, the country-rock has 
been transformed into ore by the more or less complete 
chemical replacement of its original constituents. 

FIG, 103.—DIAGRAM TO SHOW MODE OF OCCURRENCE OF BOHNERZ. 
B, Bohnerz or odlitic limonite; C, cave-earth, etc. ; L, limestone. 
(a) Masses occupying Cavities—Among the best examples 
of this type are the Bohnerz deposits which are so frequently 
met with in the Mesozoic limestones of middle Europe. 
(Fig. 103). Bohnerz is an odlitic or pisolitic limonite—the 
spherical grains of which vary in size from turnip seeds to hazel- 
nuts, and often show a concentric radiated structure. The ore 
is usually charged with many impurities, such as clay, sand, etc., 
and not infrequently contains fossil organic remains of 
Tertiary age—such as mammalian teeth and bones, together 
with plants. In most cases the formation would seem to be 
a deposit from springs; but occasionally the ironstone occurs 
