REVIEWS 441 
are no considerable ore bodies in the surrounding Archean not con- 
nected in some way with the andesite. Furthermore, no considerable 
mining district in Boulder, Gilpin, or Clear Creek counties occurs 
except where these andesites are present, even though the same 
Archean country rock is everywhere present, and in all probability 
was subjected to the action of ordinary underground waters for many 
times as long before the intrusion as after. Under these conditions 
the connection of the ore bodies with the disturbance of the under- 
ground circulation by the intrusion of the eruptive rock can hardly be 
doubted. The possibility of their being formed by normal under- 
ground circulation counts for little as against the fact that, in the long 
time from Archean to Tertiary, they were not formed, and in the rela- 
tively short time since, under the new conditions, they were. 
It is equally clear, however, from a study of the ore bodies, that 
the process of their formation occupied a long period, as measured by 
ordinary standards. The ores are associated with the most altered 
portions of the eruptives. They occupy often fissures in the latter 
which could only have been formed after the rock had consolidated 
and cooled sufficiently to be amenable to the ordinary stresses existing 
in the rocks of the surface of the earth. The halogen elements are 
conspicuously absent, and there is no known reason for inferring that 
pneumatolitic action was prominent when the ore bodies were formed. 
It may be added that the veins are typical “fissure veins” carrying 
sulphide ores ; the very class of deposits principally under discussion. 
Among the many phases of ore bodies, aside from the problem of 
their primary genesis, which are discussed in the volume under review, 
there are none more important than the secondary enrichment of the 
sulphides, and the metasomatic processes in fissure veins. The first is 
discussed in separate papers of Emmons and Weed and in portions of 
papers by Van Hise, Rickard, and de Launay. ‘The second is treated 
in considerable detail by Lindgren. 
Starting from a disposition to question Posepny’s dictum that sul- 
phides were not formed by the waters of the vadose circulation, 
Emmons, Weed, and Van Hise have independently worked out the 
process by which the sulphide ore bodies at and near the water level 
become enriched by material carried down from the oxidized ores 
above. The process depends principally upon the relative affinity of 
the various metals concerned for oxygen and sulphur, by which, for 
example, iron sulphide comes to act as a precipitant for zinc and lead 
