214 Deruy — Notes on Certain Schists of the Gold and 



these latter minerals, with their associates, form independent 

 rock masses, a possible prototype for such schists as No. I can 

 be imagined. The question still remains of the mode of ori- 

 gin of such a prototype if it exists, but the occurrence of 

 corundum in olivine rocks shows that the association of alu- 

 mina and magnesia in ultra basic eruptives is not an impossi- 

 bility. 



The probable occurrence in this rock of a soluble compound 

 of zirconia is also suggestive of a possible prototype in the 

 nepheline- or augite-syenite groups, in which such compounds 

 have thus far proved to be most frequent and in which rocks 

 with corundum indicating an excess of alumina have lately 

 been recognized. Thus far, however, no member of this 

 group at all approaching in composition the Serra de Gigante 

 rock appears to have been described. That rocks of this group 

 may have existed in the district and have contributed to the 

 metamorphic schist series, is perhaps indicated by the perof- 

 skite- and, possibly, melilite-bearing schists from the neighbor- 

 hood of Dattas. 



As to how far the above analyses are typical of the phyllites 

 of the series in question can only be conjectured. These, 

 which as regards thickness are evidently more important than 

 the quartz, iron-mica and calc-schists with which they are asso- 

 ciated, present almost universally an aspect that has led to 

 their being generally denominated as chloritic or talcose, that 

 is to say they are predominantly of micaceous texture and of 

 a character that suggests magnesian minerals. True chloritic 

 and talcose as well as amphibolitic schists undoubtedly occur 

 in the series, but it is almost certain that they are subordinate 

 to sericitic schists, agreeing more or less perfectly in character 

 with Nos. II to VIII of the above analyses. So far as can be 

 judged from a superficial examination of material which for 

 the most part is profoundly decomposed, the more quartzose 

 types represented by Nos. V. to VII are the most abundant 

 and characteristic, and these, from their composition and inti- 

 mate association with the quartz schists, may be presumed to 

 be, for the most part, of clastic origin. If so, however, the 

 original clays must have been sufficiently rich in alkalies to 

 form a sericitic mica as the predominant element of their 

 metamorphosed state. Judging from the above analyses and 

 from the almost universally micaceous character of the phyl- 

 lites of the region, the proportion of alkalies must have 

 been very uniformly above 5 per cent. 



It is interesting to compare with the proportion of alkalies 

 above deduced for these phyllites that given in the extensive 

 list of analyses of ceramic materials in vols. 16 and 18 of the 

 annual reports of the U. S. Geological Survey. Out of 550 



