96 IOWA ACADEMY OF SCIENCE 



It is not difficult to fancy the manner in which metallic substances 

 of meteoritic origin may become incorporated with ore-materials gener- 

 ally. After reaching the surface of the earth, both cosmic dust and the 

 larger meteorites must mingle witli the soil, more or less quickly oxi- 

 dize, and enter, by means of the circulating groundwaters, or other- 

 wise, 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 ore-materials. The dis- 

 tinction 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 

 proportion is regarded as coming from extra-terrestrial sources. 



Although there is probably no such universal sea of groundwater as 

 that pictured by Van Hise, there is yet no reason for not 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 mi- 

 grate, generally inward as individual particles, but occasionally or 

 spasmodically outward, in connection with volcanic flows. 



In the course of the inward migration of ore-materials temporary ore- 

 bodies are often localized, in the vadose zone, chiefly. How much of 

 these materials are of recent extra-terrestrial origin and what propor- 

 tion is the product of rock-decay, is at the present time difficult to esti- 

 mate. The meteoritic contribution has received as yet insufficient atten- 

 tion. That it may be much 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-materials now seems not unlikely. 

 It is probable that most of the diffused metallic content of the sedi- 

 mentary rocks is in reality immediately derived from meteoritic sources ; 

 for its derivation from the country-rock of mining districts, especially 

 in tracts far removed from volcanic activity, has never been a very satis- 

 factory explanation. 



As commonly regarded a mineral vein consists of (1) the deeper 

 primary portion below groundwater level, and (2), above the latter a 

 limited weathered part known as the gossan. The lower part is com- 

 posed of sulphides and the upper portion of oxides. Of late, between 

 the two, at groundwater level, there has come to be recognized a third 

 zone, that of secondary sulphide enrichment. According to this simple 

 conception the movement of ore-materials liberated by weathering and 

 passed into solution is merely slowly down the course of the vein. 



