


234 STRUCTURAL AND FIELD GEOLOGY 

traces of old land-surfaces. Lacustrine and fluviatile deposits 
of Tertiary age have now and again been preserved, under 
lava, as in California and Victoria (Australia). The auri- 
ferous gravels of these regions are believed to be river- 
gravels belonging to the Pliocene. They are often more or 
less hardened by infiltration of silica, ferruginous matter, etc., 
and constitute the “deep leads” of the miners. The shallow 
placers of the same regions are of recent age—derived in 
considerable measure from the denudation of the older series. 
The alluvial deposits of the Ural Mountains, which yield both 
gold and platinum, the “stream-tin” (cassiterite) accumula- 
tions of Cornwall, which are now practically exhausted, and 
the alluvial tin-fields of Malaysia, from which three-fourths of 
the world’s output at present come—are all examples of the 
same class of ore-formations. Placers of older date than the 
Tertiary are of rare occurrence, only a few gold-bearing con- 
clomerates having been met with in Mesozoic and Palzeozoic 
systems, and these are seldom rich enough to be worked. 
(3) Ores occurring in Schistose Rocks.—Ores of iron 
and manganese are the most frequently occurring formations 
met with as beds interstratified with schistose rocks. It is 
sometimes difficult to distinguish such syngenetic formations 
from certain epigenetic formations which are known as 
“bedded veins,” and of which some account is given in the 
sequel. Usually, however, a bedded ore is not so sharply 
marked off from the schists among which it lies, as is the 
case with a true vein. The bedded ore does not traverse 
overlying and underlying schists, nor does it send out veins. - 
It behaves, in short, like a truly contemporaneous bed— 
following all the flexures, folds, and crumplings of the series 
in which it occurs. The thickness of such beds varies 
indefinitely: they are usually lenticular, and thicken and 
thin out irregularly. From a few inches, they may gradually 
swell out to many feet or even yards. The thickest bed of 
ore yet encountered is that of Grangesberg in Sweden, which 
is not less than 100 yards. In that country the bedded ores 
are usually more or less closely associated with crystalline 
limestone, or with a rock consisting mainly of pyroxene and 
amphibole, and often containing garnet and epidote. Iron- 
ores are obtained from similar schistose rocks in Norway. 
