DIASTROPHISM AND THE FORMATIVE PROCESSES 695 
guide as the temperature curve, for, in the planetesimal earth, 
both arise from compression and its direct and indirect conse- 
quences. 
After an elaborate study of the most reliable data, Dr. H. S. 
Washington thus sums up his conclusions in a recent paper: 
‘“‘T am inclined to place the average density of the crust at about 
2.75 at least for the uppermost shell, while that of 2.80 would prob- 
ably be nearer the truth for an average of any considerable depth, 
say 20 miles or more.’”’ The mean depths of these two shells can 
scarcely be more than 8 or 10 miles apart. The rise of density in 
this little difference of depth, if projected to the earth’s center, 
would give a density there nearly twice that computed from the 
classic law of Laplace, or from the law of Roche specially formu- 
lated to meet the astronomical requirements. No account is here 
taken of mechanical compression, for the specific gravities adopted 
as the basis of the estimates were all taken under atmospheric 
pressure. Much less was any account taken of hypothetical 
quantities of metals or other specially heavy material, for both 
these shells are formed of common rock. 
The two elements most common in the zone of observation, 
oxygen and silicon, often unite to form tridimite in the outermost 
shell but not in the plutonic rocks, where the same elements appear 
as quartz. This distribution is commonly assigned to differences 
in the physical conditions of the two horizons, especially differ- 
ences in pressure. The specific gravity of tridimite ranges from 
2.28 to 2.33, while that of quartz is 2.65. There is thus a rise of 
density of 15 per cent, so far as these minerals are concerned, 
between two horizons both of which lie in the limited zone made 
accessible by deformation and denudation. 
The most instructive and suggestive data, however, are found 
in the progressive stages of increase in density developed in several 
different kinds of silts as they pass into various kinds of schists, 
and thence, in part, into.a group of heavy minerals of which the 
garnets may be taken as types. The compression of the silts into 
shale may be neglected since a notable part of the increased density 
tH. S. Washington, ‘‘The Chemistry of the Earth’s Crust,” Jour. Franklin Inst., 
December, 1920, p. 804. 
