THEIR ORIGIN AXD RELATIOXS. 27 



rocks is but imperfectly known, but the changes probably take place under 

 the influence of percolating waters. Tiiat these clian"-es are slow in manv 

 cases is rendered probable by the fact, that when rocks have been exposed 

 to rapid alterations by hot and mineral waters, tlie result is a geneial de- 

 struction of the rock mass, a disintegration of it as a whole, and not such 

 changes as are seen in rock masses in general. It is probable that both cold 

 and thermal waters have contributed to the change, as the latter are abun- 

 dant in volcanic regions at the present, and Ave have the right to infer that 

 they were so in past time at the localities in which ancient igneous activity 

 was manifested. 



- These alterations are considered to be molecular, or belonfjno- to the 

 rock mass as a whole, although some portions and some minerals are altered 

 more rapidly than others. The general tendency of rock alteration seems to 

 be the breaking up of the original constituents, and the formation of quartz 

 and other minerals, that give to the rock characters closely simulating those 

 of sedimentary rocks. 



The sediments also undergo the same changes, and in extreme cases 

 produce crystalline schists and gneisses. The changes in them are appar- 

 ently brought about by the same agencies as the changes in eruptive rocks, 

 and thermal waters may have been an important factor in producing crys- 

 talline structure in the former. 



In the various alterations of the rocks of every kind the new mineral 

 structures come apparently from the segregation of mineral matter, either 

 from the rock or adjacent sources, in some place suitable for its deposition. 

 The place may be some fissure or cavity, or it may be in the solid rock mass 

 itself, by the removal of one or more chemical constituents from the immediate 

 point of action, and the substitution of others. So f^ir as the rock mass goes 

 when no foreign material is carried into it, these changes may be defined as 

 the migration or aggregation of the chemical elements, produced by their 

 tendency to seek such unions as shall expose them under their present con- 

 ditions to less disturbing elements than their former relations did — a ten- 

 dency to pass from an luistable towards a more stable condition. The final 

 result of these changes is, usually, to produce claj-s, ochres, quartz, and car- 

 bonates; the latter of which while not stable in position, are apt to be so in 

 composition ; e. r/., calcite, while readily soluble and removed, generally 

 reappears as calcite — the position unstable, the union stable. The general 

 principle of change is the same, whether the mineral matter be leprecipitated 



