626 /. H. L. VOGT 



alnoite), but on the other hand are not — as yet (1920) — known from 

 deep-seated rocks. 



If we compare a number of analyses of troctolite ("forellen- 

 stein" and feldspar-peridotites) with for instance the analyses 

 of slags given in my work "Silikatschmelzlos," I, p. 16 (containing 

 ca. 30-42 per cent SiOz, 8-28 per cent AIO3, 20-40 per cent CaO and 

 up to 13 per cent MgO with some FeO, MnO, etc.), in which a 

 melilite mineral (melilite-gehlenite) is crystallized, it is apparent 

 that by re-melting at one atmosphere some of the deep-seated rocks 

 mentioned, which consist only of olivine and anorthite-bytownite, 

 they must recrystallize with more or less melilite besides several 

 other minerals of which one is spinel (Vol. XXIX, p. 524). 



Gehlenite occurs in some contact zones; it must consequently 

 under certain conditions be formed at high pressure. And melilite 

 occurs, as just mentioned, as a rarity in some dike rocks, which 

 might have been solidified at a tolerably great depth, thus also at a 

 tolerably great pressure. 



It is worth noticing, however, that melilite, so far as known, is 

 lacking in the common deep-seated rocks, and especially that, 

 instead of a melilite mineral, we find in the troctolites the combina- 

 tion olivine and anorthite-bytownite. 



A similar case is that of leucite which can crystallize in melts at 

 a pressure of one atmosphere, and which occurs in many effusive 

 and some dike rocks, but has only rarely been found in deep-seated 

 rocks. When microcline and biotite are melted together in certain 

 proportions, leucite results. And in many deep-seated rocks which 

 by re-melting at one atmosphere would give leucite, we find instead 

 of leucite other minerals, as microcline, biotite, etc. 



3. On the other hand there are some minerals which in part 

 preferentially and in part exclusively belong to deep-seated or other 

 rocks formed at a very high pressure. As an instance we may 

 choose garnet. 



By re-melting garnet there results, as is known, not this mineral, 

 but a mixture of other minerals, according to the composition of 

 the garnet — melilite, anorthite, olivine, spinel, etc. On the other 

 hand, we may get garnet by using certain chlorides, for instance 

 AICI3, as the solution medium. Garnet — or at least some varieties 



