266 STRUCTURAL AND FIELD GEOLOGY 
rock of various kinds. In each district where they occur they 
belong to the latest or one of the latest epochs of volcanic 
activity for that district. Hot springs, solfataras, etc., are 
frequently found near them, and even when these are absent 
the surrounding rocks are always more or less altered, as if 
they had been subjected to the action of heated water and 
vapours. The same authority would ascribe a similar origin 
to the older lead-silver veins of the Erzgebirge, the Harz, 
Kongsberg (Norway), Przibram (Bohemia), etc., and the old 
gold-quartz “ Mother Lode” in California. 
The ore-formations of Sarawak have been recently shown 
by J. Somerville Geikie to be true contact-formations. They 
include ores of iron, antimony, arsenic, zinc, lead, mercury, 
etc., and native gold and arsenic. The region is occupied 
chiefly by Mesozoic limestone and shales, which are often 
highly shattered and brecciated and saturated with silica. 
Numerous dykes and sills of quartz-porphyry traverse these 
rocks, and are supposed to proceed from a concealed batholith 
of granite. Indeed, only a few miles away from the mines 
granite comes to the surface to form considerable hills. The 
ores occur not in the form of true lodes but mostly as impregna- 
tions and disseminations in the shales, and as irregular bodies 
in the limestone. Now and again the dykes yield a small 
percentage of gold. 
Origin of Ore-Formations.—Here we refer mainly to epigenetic 
formations—the origin of syngenetic ore-formations has already been 
sufficiently discussed. We have learned that native metals and ores of 
various kinds occur as original constituents of crystalline igneous rocks— 
the ores being sometimes so abundantly developed that they can be 
profitably worked. From the researches of Sandberger and others, 
moreover, we know that minute quantities of many of the heavy metals 
have been detected in such minerals as olivine, augite, hornblende, and 
mica. Olivine, for example, has yielded iron, nickel, copper, and cobalt ; 
in augite have been detected iron, copper, and cobalt, and less frequently, 
nickel, lead, tin, and zinc, while antimony and arsenic are occasionally 
present; from hornblende have been obtained copper, arsenic, and 
cobalt, and not infrequently lead, antimony, tin, zinc, and bismuth ; lastly, 
in the micas (which are often specially rich in the heavy metals), have 
been recognised tin, arsenic, copper, bismuth, uranium, lead, zinc, silver, 
cobalt, and nickel. Doubtless, the proportion of metal present in any 
individual mineral is extremely minute, but the sum of metal contained 
in this way by the several constituents of a rock-mass must really be very 
considerable. From the phenomena connected with contact ore- 

