Presidential Address—The Geology of Devon. 565 
the vein-material comes from the sides or from below. It is reason- 
able to suppose that both sources may have contributed to the 
supply, though in certain cases a change in the deposits, accompany- 
ing a change in the country rocks, would seem. rather to favour the 
notion of lateral secretion. Thus Mr. Phillips remarks, with regard 
to the Tavistock district, that the copper-ores are often associated 
with a blue clay-slate. If the slate becomes deeper in colour, iron- 
pyrites alone occurs; and if the rock becomes quartzose, even the 
pyrites disappear. 
Whilst endeavouring to trace the source of the ores in metal- 
liferous lodes we should bear in mind the experiments of Sand- 
berger, who found that the heavy metals occur in the silicates of 
the crystalline rocks of every age. Augite and the magnesia-micas 
are especially rich, and the lithia-micas are noted as being stan- 
niferous. ‘The origin of tin-ores is probably different to that of the 
sulphuretted ores, though both are often best developed at the 
junction of igneous and sedimentary masyes. This, of course, is 
partly accounted for by greater facilities for fissuring, and still more 
by an increase of heat, which is likely to promote the underground 
circulation, and above all to increase the solvent power of under- 
ground water. 
The question of solution has always been a difficult one, and has 
inclined some people to adopt the notion of sublimation of the 
metallic sulphides. As an alternative theory we have had the 
reduction of sulphates by organic agency. But people are begin- 
ning to think that both these agencies may be dispensed with, and 
that, under peculiar conditions of heat, pressure, and dissolved gases 
and salts, the solvent powers of water may be largely increased. 
Anyhow, it is perfectly certain that metallic sulphides, such as 
cinnabar and pyrites, are being deposited from hot springs along 
with various forms of silica, both in California, and at Steamboat 
Springs in the State of Nevada. We may well believe that this 
latter place, of which an account was given in the Quarterly Journal 
of the Geological Society as long ago as 1864, represents with a 
certain amount of fidelity the conditions which prevailed in the 
upper portions of the metalliferous lodes of Devon and Cornwall 
during a period, not of maximum activity, but when a considerable 
deposit was taking place. 
The solution and transport of tin-ores are capable of a different 
explanation. As is well known stanniferous deposits are not only 
very local, but are also accompanied by a peculiar group of minerals, 
such as topaz, schorl, axinite, and fluor, which contain a notable 
quantity of either Fluorine or Boron, and in the case of schorl of 
both these elements. Daubrée observed that this is the case 
wherever tin-ore has been found, and he suggested that, in the 
first instance, tin was brought up from what he calls the general 
reservoir of the heavy metals as a fluoride. The interesting 
chemical experiments connected with this ingenious hypothesis are 
detailed in his great work on Experimental Geology. According 
to these views stannic-fluoride and steam would decompose each 
