THEIR CLASSIFICATION 29 



dissipation of energy ; while the chemical elements or the molecular combi- 

 nations tend to seek new unions better adapted to the present circumstances 

 of the rocks. These changes progress, going on from one form to another. 

 It is not uncommon to find that as steps in the progress, the original miner- 

 als and glass are altered to other more or less well defined minerals, having 

 sometimes perfect crystalline form; while these in their turn yield to the act- 

 ing forces, and new mineral combinations are entered into, and so on down- 

 ward in the course of the alteration. It may be said that an eruptive rock, 

 when it has passed from the interior to the exterior of the earth, becomes 

 a chemical laboratory, in which solutions, reactions, and precipitations are 

 continually carried on — experiment after experhnent, change after change, 

 succeed one another, according to the materials, reagents, and conditions. 

 But they always progress in one direction ; the combination last formed is 

 always more stable in the then conditions than the preceding combinations 

 were ; with any change of condition there would of course come change in 

 relative stability. 



The induration or hardening of rocks would thus oftentimes be no index 

 of exposure to heat, for if the mineral formed or infiltrated into the rock 

 mass is one which stands high on the scale of hardness, e. g. quartz, then 

 induration follows as a matter of course. 



From the principle of passage or unstableness it follows that the glassy 

 state is nearest the prinritive condition, and is to be looked upon as the 

 starting point of the indigenous and secondary minerals in rocks ; hence it 

 should come first in our study and be traced in its process of crystallization 

 and alteration. 



The three classes of products above discussed will be mentioned hereafter 

 as products of the first, second, and third class, or as, 1st, foreign; 2d, indi- 

 genous; and 3d, alteration or secondary products. 



The two first classes have been collectively and singly called by the 

 writer original in contradistinction to the secondary products. 



Cases of envelopment occur in minerals of the second class frequently, 

 but they can be distinguished as easily under the microscope, in rocks not 

 too far altered, from the foreign or secondary minei'als, as a coarse conglom- 

 erate can be distinguished from a granite by the naked eye, or as a piece of 

 wood joined to another by a mortise and tenon can be distinguished from 

 the natural growth of a limb. 



The various chany-es that rocks undergo in their alterations are determined 



