REVIEWS. 
Influence of Country Rock on Mineral Veins. By WALTER HARVEY 
WEED, United States Geological Survey, from the transac- 
tions of the American Institute of Mining Engineers, Mexi- 
can meeting, IgOl. 
ABUNDANT evidence is brought forth and satisfactorily stated in this 
paper to show three things: that the structural characters of vein 
fissures are affected by the country rock; that the mineral contents of 
ore deposits formed by metasomatic replacement vary with the nature 
of the enclosing rock; that no invariable relation can be established 
between rock types and ore deposits. Variation in structure of the 
fissure naturally, though not invariably, follows a change in texture, 
cleavage, and hardness of the rock through which it passes. In rocks 
which are fissile or easily fractured, fissures often lose their definite 
character and ramify the rock through which they pass, so that the ores 
which they carry lose their economic significance. In passing from 
soft to hard rock mineral veins are apt to grow narrow, and are often 
deflected. Regardless of the origin of the ore, there seems to bea 
connection between the nature of the mineral deposit and the country 
rock, where the mineral deposit is of the nature of a replacement. No 
such connection appears in the filling of open fissures. On account of 
differences in chemical constitution of the ore-forming solutions, the 
chemical reactions which took place in the process of vein forming in 
a rock of a given kind in different districts, were necessarily different, 
resulting in a variety of ore deposits. Within a limited district, how- 
ever, the nature of the ore forming solutions was often very constant, 
and there is a uniform variation in vein content, with variations in the 
wall rock. 
These generalizations are supported by an interesting body of evi- 
dence drawn from the literature on ore deposits, both American and 
German, and from the writer’s personal observations. The principles 
deduced are moderate, and are for this reason all the more acceptable. 
They are an advance on anything that has been attempted along this 
line, and have both theoretical and practical value. 
FRANK A. WILDER. 
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