1891. ] DERBY—ON ACCESSORIES OF ROCKS. 203 
amount of wear in the formation of sedimentaries from eruptives, so 
that it may be said that, when occurring in the former, these elements 
are still in the original packages. Moreover, in virtue of their high 
specifig gravity (near 5) they become concentrated in the coarser 
arenaceous deposits, being almost completely sifted out of the finer 
argillaceous ones, as can be verified by a comparative wash in any sand 
and mud bank. Unless, therefore, these rare chemical elements are 
introduced into the mass subject to metamorphism by the action of the 
so-called mineralizing agents, (as fluorine, boron and tin are supposed 
to be in the formation of tourmaline, topaz and cassiterite,) it is difficult 
to conceive how the minerals in question can appear as new formed 
elements in a metamorphosed sedimentary. Their early crystallization 
and uniform distribution in eruptives, as well as their absence from 
schists metamorphosed by contact (in the rare cases in which zircon has 
been noticed it may be presumed to have existed in the original 
sediment) exclude the hypothesis of such an introduction. 
On the hypothesis of a sedimentary origin for the crystalline schists 
that so frequently contain zircon alone, or zircon and monazite, we 
should expect these minerals to show more or less distinct signs of wear 
and to characterize quartzose rather than feldspathic rocks. The 
observations thus far made that bear upon this point are as follows : 
schists free from quartz, such as amphibolites and amphibole schist, 
often show an abundance of perfectly, sharply defined and fresh looking 
zircons ; staurolite-bearing argillaceous and micaceous schists often 
fail to show zircon, or give only a few grains, usually with a worn 
appearance ; the Brazilian gneisses and feldspathic mica schists have 
never failed, so far as tested, to show zircon, more frequently than 
otherwise associated with monazite, in about the same abundance 
and with the same perfection of form and lustre as is found in the 
typical granites.* 
The first of these observations is contrary to the antecedent prob- 
ability above established, but in harmony with the studies of Lossen, 
Williams and others, as well as with the field observations in Brazil, 
tending to prove the derivation of many of the amphibole schists from 
eruptives. The second case, in which the schists are unquestionably 
of sedimentary origin, confirms the a frtord reasoning regarding the 
probabilities of the appearance of minerals of the rare elements in 
metamorphosed sediments. The third observation is in accord with 
an opinion now held by a number of able geologists that many of the 
*Chroustschoff’s observation (Tschermak’s Mittheilungen, VII, 1886) that granite is character- 
ized by sharp angled and gneiss by rounded zircons, does not hold good when a larger series is examined. 
In both rocks perfectly sharp angled crystals are the exception rather than the rule, and apparently 
characterize the amphibolic rather than the micaceous types. 
