348 
MINING GEOLOGY 
state, either through magmatic secretion or by expulsion of the 
volatile compounds of the metals during the progress of magma¬ 
cooling; and (4) derivation of metallic particles from extra-ter¬ 
restrial sources, and their later segregation through the action of 
percolating surface-waters. Of these several groups contention 
regarding the first mentioned is now obsolete; the conceptions 
concerning the second and third enter into nearly all of the 
recent discussions of the subject; the idea of the last receives yet 
only incidental attention, but it is likely to prove the most impor¬ 
tant of all. 
It is not difficult to fancy the manner in which metallic sub¬ 
stances of meteoric origin may become incorporated with ore- 
materials generally. After reaching the surface of the earth, 
both cosmic dust and the larger meteorites must mingle with the 
soil, more or less quickly oxidize, and enter, by means of the 
circulating groundwaters, or otherwise, the deep-seated zone, 
in the same way as any of the heavier mineral particles liberated 
from the surface rocks through decomposition are supposed to do. 
The processes involved are essentially the same as for the changes 
and movements of rock-forming minerals. The distinction to be 
made is that, instead of the ore-materials being derived from the 
breaking down of the rocks of the lithosphere, a very large pro¬ 
portion is regarded as coming from extra-terrestrial sources. 
Although there is probably no such universal sea of ground- 
water as that pictured by Van Hise, there is yet no reason for 
believing that surface-water readily penetrates to the deep-region, 
even to the zone of rock-flowage. The lithosphere thus represents 
merely the flotsam and jetsam of the globe, through which the 
heavier materials may migrate, generally inward as individual 
particles, but occasionally or spasmodically outward, in connec¬ 
tion with volcanic flows. 
In the course of the inward migration of ore-materials tem¬ 
porary ore-bodies are often localized, in the vadose zone, chiefly. 
How much of these materials are of recent extra-terrestrial origin 
and what proportion is the product of rock-decay, is at the present 
time difficult to estimate. The meteoric contribution has received 
as yet insufficient attention. That it may be more important than 
has been suspected hitherto is clearly shown by recent observations 
in desert regions. That this is the main source of vadose ore- 
