O. A. Derby — Notes on Monazite. 221 



Occurrence in basic rocks. — Up to a very recent period the 

 rule seemed to be very firmly established that it was useless to 

 look for monazite as an autigenetic element in any but highly 

 acid rocks. In the scores of rocks examined, both sound and 

 decomposed, it had been found to be almost universal in the 

 muscovite granites, or their porphyritic and gneissic equiva- 

 lents ; frequent in the biotite granites, but lacking in the 

 amphibole-granites and all other more basic rocks that had been 

 examined. The monazite-bearing schists above mentioned from 

 Sopa, Sao Joao da Chapada and Serra do Gigante, however, 

 indicate that this, rule is not general, though as the type of 

 mineral that they present is peculiar and perhaps of secondary 

 origin, it may be that it only requires to be somewhat modified 

 rather than set entirely aside. The Sopa schist (that one free 

 from zircon) have not been analyzed, but it is evidently com- 

 posed almost exclusively of sericite and there is no difficulty in 

 considering it as a modified porphyry. That of Sao Joao da 

 Chapada, though basic in the low silica (37*77 per cent) and 

 high iron (28'36 per cent, for the complete analysis see No. II 

 of the preceding paper) contents, may, as I attempted to show 

 in the paper above cited, be plausibly considered as a basic 

 phase of a mixed dike of essentially granitic character. No 

 such hypothesis is, however, possible with the Serra do Gigante 

 schist with its 38 - 32 per cent of silica, 28 - 16 per cent of alumina 

 and 12*04: per cent of magnesia. As already stated under the 

 head of inclusions, all the elements of this schist are apparently 

 secondary and it might therefore be considered as a metamor- 

 phosed sedimentary clay. The high percentage of magnesia is, 

 however, anomalous for such a clay and in the preceding paper 

 in which this rock is more fully discussed the hypothesis of 

 the decomposition and leaching, prior to metamorphism, of an 

 original eruptive rock is suggested. Even so, however, it seems 

 necessary to imagine a type of eruptives that has not yet been 

 clearly recognized though there are indications of its possible 

 existence. 



