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572 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. — 
more basic kinds, as basalts, diorites, olivine rocks, &c., alteration of 
this nature may be studied in all its stages. (See pp. 107, 331.) 
But mere alteration by decay is not what geologists denote by 
metamorphism. The term has been, indeed, much too loosely 
employed; but it is now generally used to express a change in the 
mineralogical or chemical composition and internal structure of 
rocks, produced at some depth from the surface through the operation 
of heat, and heated water or vapour. A metamorphic rock may be 
as compact and crystalline as the parent mass from which it has been 
altered, like which, also, when exposed at the surface, it again under- 
goes alteration by weathering. 
‘Metamorphism may be effected: Ist. By the action of heated 
water carrying carbonic acid and mineral solutions produced by 
carbonic or other acid (p. 300); 2nd. By the action of hot vapours 
and gases (pp. 235, 297); 3rd. By the heat generated in the crush- 
ing of rock-masses during contraction of the terrestrial crust (p. 290) ; 
4th. By the intrusion of heated eruptive rocks, sometimes containing 
a large proportion of absorbed water, vapours, or gases (p. 541 seq.) ; _ 
5th. Occasionally and very locally by the combustion of beds of coal. 
Metamorphism is manifested in two distinct phases. Ist. 
Local (the metamorphism of contact or of juxtaposition), where 
the change has been effected only within a limited area beyond 
which the ordinary condition of the altered rocks can be seen. 2nd. 
Regional (normal), where the change has taken place over a large 
tract, the original characters of the altered rocks being more or less 
completely effaced. 
§ II. Local Metamorphism (metamorphism of contact or 
juxtaposition). 
The influence of thermal waters in effecting mineralogical changes 
within rocks has been already described, and some illustrative ex- 
amples have been given (pp. 299, 309). Such changes may take place 
along the sides of the channels in which the heated water makes its 
way to the surface, and as far into the rock around as the water may 
be able to penetrate. Hruptive rocks, also, when intruded among 
limestones, sandstones, shales, and other sedimentary formations, 
produce in them various kinds and degrees of alteration. 
Bleaching is well seen at the surface, where heated volcanic 
vapours rise through tuffs or lavas and convert them into white 
clays (p. 235). Decoloration, however, has proceeded also under- 
neath, along the sides of dykes (p. 553). ‘Thus in Arran a zone of 
decoloration ranging from 5 or 6 to 25 or 80 feet in width, runs in 
the red sandstone along each side of many of the abundant basalt 
dykes. This removal of the colouring peroxide may have been 
effected by the prolonged escape of hot vapours from the cooling 
lava of the dykes. Had it been due merely to the reducing effect of 
organic matter in the meteoric water filtering down each side of the 
