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spectral lines on the temperatures in various levels of the sun's
atmosphere. The abundances of the elements in many of the
stars are very similar to these values for the sun though
Important differences occur. Also the abundances in planetary
,• nebulae are very similar to stellar values. Though for the
most part we are really discussing solar quantities, it will
be assumed that all sources are sufficiently similar so that
numerical values can be compared.
The proportions of the elements in the earth's crust
have obviously been modified in several ways. During the process
of formation^the earth lost most of the most volatile elements,
hydrogen, the inert gases, carbon as CHw, nitrogen as NH3 or Ng,
oxygen as HgO, and possibly some proportions of other elements
though such loss is not evident (Urey, 1954 b). There has been
a marked differentiation of the surface regions by partial
melting and crystallization processes and a loss of the
slde3?ophile and chalcophile elements to the deep interior.
Erosion by water has further differentiated the surface regions.
It is exceedingly difficult to estimate in any reliable way
what the mean composition of the surface region of the earth
is, yet there are some data of use to the present study.
It is generally assumed that meteoritic matter, since the
time it formed from solar material, has undergone less chemical
fractionation than any terrestrial material found on the surface of
the earth.
The type of fractionation that is recognizable in