i88 The American Geologist. September. 1902 
Miifaniiliar with petrographic distinctions; and, as is well 
known, shales, limestones and sandstones grade into one an- 
other. For the careful study of nietasomatic rejilacement, 
which must be made to establish scientifically a relation be- 
tween country- rock and vein-filling', finer distinctions must be 
made. The granitic rocks of some writers include gabbros, 
diorytes, granite and aplyte, with a wide range of mineral and 
chemical compositions. The most that can be attempted at 
present is to present the known facts of occurrence in deposits 
about which there can be no doubt, or which have been care- 
fully studied. The facts here set forth show the advantages of 
geological examination of the district about a mine, especially 
of the area containing the vein. It is evident that it is im- 
portant to ascertain the extent laterally and the probable extent 
vertically of the rock in which the ore occurs ; and, if other 
rocks occur, what they are and what eiTect, if any, they will 
have on the vein-fissure and vein-filling. Such associations 
have long been recognized in a rough way by the miners, who 
say "that mineral will not live long in such a rock." The in- 
stances noted in the literature of ore-deposits are few, compared 
to those actuallv encountered in mining operations, where the 
character of the vein has changed in depth. Such change is, 
it is true, very often due to secondary alteration, with or with- 
out rcconcentration and enricliment of material, but in many 
cases it is probably due to change of rock. 
Conclusions. 
From the evidence ]:)resented the following conclusions are 
drawn : 
1. The structural characters of vein-fissures, such as course, 
width, etc., vary with the nature of the country-rock. 
2. The mineral contents of veins formed wholly by the fill- 
ing of open fissures are not ati'ected by the nature of the vein- 
walls. 
3. The mineral contents of ore-deposits formed by nietaso- 
matic re]:)lacement vary with the nature of the enclosing rock. 
4. As nietasomatic processes vary in character with the na- 
ture of the solutions, no invariable general relation can be 
established between certain rock-types and rich ore-deposits. 
