272 STRUCTURAL AND FIELD GEOLOGY 

of all kinds, they would at the same time insinuate themselves into the 
narrowest and closest crevices, and permeate the pores and capillaries 
of the rocks themselves. The various mineral constituents of the rocks 
would thus become altered, and substances which are practically insoluble 
at the earth’s surface would be taken up. The ascensionist, therefore, 
pictures to himself such highly heated solutions not only rising through 
fissures, but being forced under pressure to penetrate more or less deeply 
the country-rock on either side—thus producing the phenomena of 
replacement and dissemination. As the water ascends to higher and 
higher levels, it will continue to deposit mineral matter since its solvent 
power must become successively diminished by decreasing temperature 
and pressure. The constituents of the ores and veinstones formed in this 
way, having usually been carried great distances, will bear no genetic 
relation to the country-rock on either side of a lode, and will not 
therefore be influenced by the nature of its walls. To this action of 
ascending water we must add that of water descending from above, 
which tends to dissolve mineral matter from rocks near the surface, and 
finding its way into fissures, must mingle with the water coming from 
below, and modify the nature of the mineral depositions that take 
place. 
The ascension theory, like its rival the theory of lateral secretion, 
gives a reasonable explanation of so wide a range of phenomena that 
it has met with much acceptance. The two theories are really not 
antagonistic—the one merely supplements the other, although it must 
be admitted that the great majority of ore-formations, other than those 
of sedimentary origin and those due to magmatic segregation and 
pneumatolytic action, are deposits from heated water ascending from 
plutonic depths. The probabilities are that the metals of ore-formations 
have been derived in part directly from molten magmas, and in part by 
secretion from rocks of various kinds, usually at a high temperature and 
under great pressure, and therefore at very considerable depths, from 
which they have been carried upwards by ascending piutonic waters.* 
Secretion, however, has not been confined to great depths, nor has it 
been effected by plutonic waters alone. On the contrary, it must have 
taken place at many different levels—at every depth, indeed, to which 
meteoric water can make its way—and thus the contents of lodes have 
been influenced again and again by solutions derived from the country- 
rock at various horizons. 

* It would appear, therefore, that no hard-and-fast line can be drawn 
between pneumatolytic ore-formations and secretionary ore-formations— 
there will be a passage upwards from the one into the other. 
