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IOWA ACADEMY OP SCIENCE 
wliicli from space they drop. Local conditions at or near the surface of 
the earth may reverse the nsnal direction of this migration so as to bring 
together the metallic materials into ore-bodies. 
' As aptly noted by Stallo, the general doctrine of meteoric agglomera- 
tion is in effect nothing more than a new statement of the law of parsi- 
mony, which forbids the unnecessary multiplication of explanatory ele- 
ments and agencies. Exemplification of the logical principle has long 
been afforded by the various branches of science, and conspicuously by 
the new geology. The past history of the earth is accounted for in terms 
of what is continually going on around us. At no stage of the earth’s 
record does genetic geology attempt to call into action forces other than 
those which are now at work changing the existing features of our globe. 
In its main features Meyer’s theory of meteoritic agglomeration is 
essentially identical with the planetesimal hypothesis of earth origin as 
recently and specifically set forth by Chamberlin."^ Upon ultimate an- 
alysis, the meteoritic hypothesis is not so wholly novel and so radically 
distinct from the nebular hypothesis of Laplace as some of its advocates 
would have us believe. G. H. Darwint . has shown that the meteoric 
swarm is dynamically analogous to a gas ; and in reality the laws of gases 
strictly apply to it. 
At the present time the planetesimal hypothesis has especial attraction 
in its bearing upon the ultimate origin of the ores. It explains satis- 
factorily many phases of ore-genesis which have long remained enig- 
matical. It does away with the sweeping claim that ores owe their form- 
ation entirely to volcanic activities ; and it suggests the vadose zone as the 
seat of the principal segregation of ore materials generally. 
The meteoritic augmentation to the earth seems to be very much larger 
than it was once supposed to be. Something of the larger meteoric irons 
and stones has long been known; and our prevailing notions of extra,- 
terrestrial materials are mainly confined to these occurences. It is, how- 
ever the constant and almost inappreciable shower of cosmic dust and 
particles falling upon the earth’s surface that is of greatest consequence 
as a possible source of ore-supply. 
The magnitude and persistency of the stellar dust shower ordinarily 
escapes notice. It is rendered visible in various ways. Hailstones are 
frequently found containing small particles of presumably meteoric iron. 
By the melting of snow in the arctic regions fine metallic particles com- 
posed mainly of iron, nickel, cobalt, etc., are obtained. 
^Carnegie Institute Yearbook, No. 3, p. 208, 1905. 
fPhilos. Trans. Royal Soc. London, Vol. CLXXX, pp. 1-69, 1889. 
