
ALTERATION AND METAMORPHISM 219 
Vogt remarks, “we find a characteristic pneumatolytic metamorphism 
of the country-rock. Each class has in abundance a halogen element, 
the tin-veins carrying fluorine (with a very little chlorine), and the 
apatite-veins chlorine (with a very little fluorine)” He concludes, 
therefore, that the materials of the apatite-veins have been extracted 
from the gabbro magma, just in the same way as the contents of the tin- 
veins have been obtained from granite. In the former case, an aqueous 
hydrochloric solution has been concerned in the extraction process, while 
in the latter case this process has been based chiefly upon a reaction in the 
presence of water of hydrofluoric acid dissolved in the granite magma. 
Not improbably, many other veins, rich in ores of various kinds, 
which occur in close association with eruptive rocks, have originated in 
the same way as the tin-veins and apatite-veins. The veins referred to 
are usually independent of the character of the rocks they traverse, 
while a more or less clear genetic connection can be established between 
them and the eruptive masses. Moreover, the rocks in which they occur 
are always metamorphosed in a less or greater degree; they have 
obviously been permeated by mineralising agents, or subjected to a 
kind of solfataric action. (See further under “ ORE-FORMATIONS.”) 
The following conclusions appear to be well established as 
a result of the study of Thermal or Contact Metamorphism :— 
1. Rocks of all kinds are liable to become metamorphosed 
at their contact with eruptives—the nature of the changes 
depending partly on the chemical composition of the invaded 
rocks, and partly on the petrographical character and the 
volume of the intrusive masses. 
2. Metamorphism has usually been effected without any 
marked alteration of the chemical composition of the rocks 
attacked. 
3. In certain cases, however, highly heated solutions, 
derived from plutonic intrusions, have penetrated and per- 
meated contiguous and surrounding rocks, and thus, by 
introducing new materials, have altered more or less con- 
siderably their chemical composition. 
4. Crystallisation has been superinduced by meta- 
morphism in derivative rocks, while igneous and schistose 
rocks have in like manner been recrystallised. 
5. The production of new minerals is a common accom- 
paniment of thermal metamorphism. 
6. Now and again the rocks near their contact with a 
batholith may be rendered schistose, owing to the develop- 
ment of new minerals along pre-existing planes of division, 
whether planes of bedding, cleavage, or foliation. 
