


ORE-FORMATIONS 269 
action of circulating water. The materials of these formations have 
been derived partly from molten magmas, and partly from the disintegra- 
tion and decomposition of rock-masses of all kinds, and have been 
carried in solution and subsequently deposited as chemical precipitates 
in pores and cavities of every shape, size, and origin. The group 
would include all cases of metasomatic replacement, impregnation, and 
dissemination. 
4. Sedimentary Ore-formation.—Under this head would come all 
ore-deposits which have originated at the surface, however deeply in 
many cases they may be now buried, and however much they may have 
been modified. Here we should group precipitates from aqueous solution 
formed in lakes, etc., and clastic ore-formations of every kind, whether 
now occupying a superficial position or occurring as beds interstratified 
with sedimentary strata of any age. Many of the ore-formations truly 
interbedded with schistose rocks would be similarly placed in this 
division. 
The mode of formation of the ore-deposits included in groups I, 2, and 
4 is sufficiently obvious and need not be further discussed. The precise 
origin of many secretionary ore-formations, on the other hand, is often 
obscure, and has been a fruitful subject of controversy. Many different 
explanations of their phenomena have been advanced, but of these we 
need only refer to the two which are at present most in vogue, namely, 
the theories of (a) /ateral secretion and (b) ascension. 
It has long been noted that the mineral contents of a lode are 
not infrequently influenced by the character of the country-rock it 
traverses. Thus one andthe same lode may be productive while passing 
through some particular kind of rock, and unproductive when certain 
other kinds of rock form its walls. In Cumberland, for example, the 
lead veins are usually highly productive when traversing limestone, but 
barren when the country-rock is slate. So again,in Derbyshire, the lead- 
veins generally carry ore when the walls are limestone, while little or no 
ore appears'in those parts of the lodes which pass through the “toad- 
stones” (Zod¢, dead, or unproductive), a local name for certain more or 
less decomposed igneous rocks. This apparent relation between lodes 
and their country-rock had been variously explained before it began to be 
suspected that the contents of the veins might possibly have been derived 
by a kind of lateral secretion from the adjacent rocks. If the materials 
had originally been diffused through these rocks, it was conceivable that 
circulating water might have leached them out and redeposited them in 
open fissures, etc. The researches of Sandberger showed that this 
suspicion or conjecture was in many cases at least well founded, for, 
as mentioned above, he obtained traces of not a few of the heavy metals 
in the minerals of igneous rocks, and also in those of gneiss. He further 
showed that silica, as well as lime and baryta, compounds of which are 
so commonly present in lodes, might quite well be derived from several 
of the original mineral constituents of igneous rocks. So that from the 
decomposition of such rocks, materials for the formation of many kinds 
of ore and of the accompanying veinstones might be supplied. He put 
