Bock Alterations. 
509 
them are taken nearly all of the valuable metalliferous ores. 
Our stores of zinc, of lead, of copper, of silver, of gold, and of 
iron, are nearly all produced by water concentration, either in 
openings in the rocks or else as replacements of their materials. 
It is to be remembered that the original materials of the earth 
are igneous, and it is only rarely that these contain a sufficient 
quantity of the useful metals to be available. 
At a given place the conditions may be such that minerals are 
being dissolved at one time and being deposited at another. 
Some minerals may be dissolved at the same time other min¬ 
erals are being deposited. In the same cavity a mineral may be 
in the process of solution in one part and in the process of 
deposition in another. In the same opening a dozen different 
combinations of minerals may be deposited in succession. The 
conditions for the deposition of the later minerals may be favor¬ 
able for the solution of those earlier deposited. All of these 
phenomena and many others are illustrated in the clefts and 
caves bearing lead and zinc in the southwestern part of our own 
state. 
From the foregoing it should not be concluded that minerals 
have any such mobility as has life. The transformations of 
minerals in most cases is exceedingly slow. Where the envir¬ 
onment remains the same as that in which the minerals devel¬ 
oped, they may continue the same indefinitely, for under given 
conditions minerals form which are stable under those condi¬ 
tions. The meteoric stones which fall to the earth may have 
had the same mineral composition which they now possess since 
their segregation, perhaps before the history of the solar system 
began. But when the conditions of environment are profoundly 
changed, as for instance when the meteoric stones leave the inter¬ 
planetary or interstellar spaces and fall to the surface of the earth, 
mineral transformations will begin. Within the outer part of the 
earth itself the most important changes in environment are 
those due to orogenic movements or to erosion. 
At first a mineral may crystallize in a lava bed, or in a deep- 
seated intrusive. The physical and chemical forces may now at¬ 
tack it. By the physical forces, it may be mashed, and from one 
mineral particle may be produced a multitude of particles. The 
