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PACIFIC SCIENCE, Vol. XVII, October 1963 
with the opinion of J. F. Lovering ( 1958 ) , V. V. 
Belousov ( 1960b), and others, that a change of 
conditions (chiefly pressure, rather than tem- 
perature) leads to a shift of the Mohorovicic 
discontinuity, with the formation of eclogites at 
the expense of gabbroic rocks of the "basalt” 
layer. The total thickness of the basic layer, 
however, is probably not more than 30-50 km. 
Thus the eclogite layer is not to be found all 
over the globe, but only in the zones of higher 
pressure. In some cases, the basalt layer has been 
fully eclogitized and has entirely disappeared, 
and the sialic layer has come into direct contact 
with the Mohorovicic discontinuity. We can ap- 
proach the problem of distribution of the sub- 
crustal eclogite layer by comparing geophysical 
and geologic data: the distribution of kimberlite, 
the appearance of eclogite inclusions in effusives, 
and, partly, the distribution of rocks formed at 
high pressures in the zone of metamorphism, 
such as kyanite schists, eclogites, jadeite (taking 
into consideration possible changes over periods 
of time). 
In the zones of normal or lower than normal 
pressure, the pressure of about 15 kilobars, 
which is necessary for the formation of eclogites, 
is reached at a depth of about 60 km, which is, 
as a rule, below the boundary separating the 
basic and ultrabasic rocks. Garnet peridotites, 
or some interlayers of eclogites that are close to 
ultrabasic rocks in composition, may be present 
there. In such cases the Mohorovicic discon- 
tinuity evidently corresponds to the true com- 
positional border between the basalt and the 
peridotite layers, not to a phase transformation. 
The isobar of the limit of possible diamond 
formation (40 kilobars) is, of course, well below 
that level. In the areas of normal or lower pres- 
sures, it must be below 120 km. In the authors 
opinion, the penetration of magma from such 
a depth is unlikely. Still less likely is the preser- 
vation of the diamond, even if magma chambers 
have formed at such depths. 
In connection with this problem it is inter- 
esting to compare the data on the finding of 
diamonds in meteorites. As far as is known, dia- 
monds have been discovered in stone meteorites 
of ureilite type (first in the Novourei meteorite) 
and in some iron meteorites. The author quite 
agrees with the opinion of Urey (1954, 1957) 
that the presence of diamonds there is evidence 
of the formation of these meteorites by the 
breakdown of some celestial body, as big as the 
moon or bigger, in which the pressure was high 
enough for the formation of diamonds. Thus, 
we cannot agree with A. P. Vinogradov’s (1959) 
opinion that achondrites were formed by the 
breakdown of small celestial bodies. The para- 
genetic associations characteristic of eclogites, 
and specifically pyrope itself, have not been 
found in the meteorites, however. This shows 
that in the basalt shell of the disintegrated body 
pressures nowhere reached 15 kilobars. This 
fact, together with the absence of meteorites of 
granitic composition, suggests that the body 
probably was smaller than the earth and less 
differentiated. This agrees with ideas which have 
already been developed on other grounds by A. 
N. Zavaritski ( 1943 ) . The above-mentioned 
data do not, however, prove that all meteorites 
have had the same origin and resulted from the 
breakdown of one planet. 
Various meteorites are still being searched for 
diamonds. This search is certainly of great in- 
terest, but there is little likelihood that diamonds 
will be found in other types of meteorites, par- 
ticularly in chondrites. Without going into de- 
tails on the hypotheses of the formation of 
chondrites, we are quite certain that in the later 
stages of existence of these meteorites the tem- 
perature was high enough so that diamonds 
would have turned into graphite even if they 
had existed. If, however, we should succeed in 
finding pseudomorphs of diamonds in chon- 
drites, as we have in some iron meteorites, this 
would be a direct proof of the formation of 
chondrites by the breakdown of some big celes- 
tial body. 
On the other hand, the discovery of graphite 
pseudomorphs of diamonds in iron meteorites 
shows that, after the breakdown of the parent 
planet, the temperatures of the meteorites were 
greater than 1200 G The preservation of dia- 
monds in the Canyon Diablo octahedrite sug- 
gests that the temperature of that meteorite at 
the time of the breakdown was below 1200 C, 
which means that it was not melted. 
The general questions discussed here are, of 
course, still open to argument. Already, however, 
on the basis of available petrographic and min- 
