ORE DEPOSIT THEORIES. 220 
These deposits always occur within the zone of contact 
metamorphism, either at or near the contact of plutonic 
eruptive rocks with the adjoining sedimentaries, or within 
the eruptive mass, but in the latter case they are always 
found attached to fragments of the country-rock which have 
become included in the magma. The most important 
criteria for recognising these contact deposits is to be feund 
in their mineralogical composition, namely, in the presence 
of minerals which we already know to be formed by contact 
metamorphic agencies. Where, as is frequently the case, 
the ores have been metasomatically deposited in limestones, 
dolomites, or other calcareous rocks, we find as contact 
minerals, garnet, epidote, light-coloured pyroxene, wollas- 
tonite, vesuvianite, and other lime silicates; whereas ore 
concentrations in slates, &c., are accompanied by chlorite, 
andalusite, chiastotite, cordierite, scapolite, &c. 
The proof of their direct derivation from the eruptive 
rocks is based on the following facts:—The deposits in- 
variably occur within the zone of contact metamorphism, 
great numbers of them lying directly at the contact of 
the eruptive rock, and some lying within the eruptive 
rock itself; but in that case they are always attached to 
fragments of the country-rock which have been included in 
the magma. The latter kind of occurrence disproves any 
lateral secretion theory whereby it might be supposed that 
the metallic contents were derived from the sedimentary 
rocks. The included fragments to which the deposits are 
attached are too small to yield the amount of ore which is 
often found attached to them. 
The fact that the ore deposits are never found within 
eruptive rocks, except where the latter contain fragments 
of the neighbouring country-rock, proves that the deposits 
were formed before the granite solidified, for otherwise we 
would expect to find them, like tin veins and apatite veins, 
which will be next considered, within the eruptive rock 
itself. 
The most common metallic constituent of these contact 
metamorphic deposits is magnetite, but sulphide ores are 
nearly always present in smaller quantities; of these, pyrr- 
hotite, iron, and copper pyrites are the most abundant, and 
after these, zinc-blende and galena. Moderate quantities of 
gold and silver are generally present, and often a trace of tin. 
In shape, the deposits vary very greatly. They very often 
form regular lodes, running either parallel to the strata or 
crossing it at an angle, the former being the most common. 
Sometimes they form irregular lenticular masses, and in 
other cases they follow the contact of the eruptive rocks. 
