356 0. A. Derby — Association of Argillaceous JRocfcs, etc. 



bead. Isolated grains gave also a very satisfactory microchemi- 

 cal reaction for phosphoric acid and a somewhat doubtful one for 

 cerium, but no definite reaction for the latter could be obtained 

 by the blowpipe method, probably on account of the smallness 

 of the quantity of pure material available or of some disturbing 

 admixture. On the strength of these observations and of the 

 close resemblance in form and optical properties with the min- 

 eral from the Sopa clay, the original determination as monazite 

 is maintained. The Sopa mineral is undoubtedly a cerium 

 phosphate and apparently agrees in form with submacroscopic 

 crystals from the diamond concentrates on which the angles of 

 monazite were verified. It appears, however, to carry some 

 titanium and lime, and possibly this peculiar type of monazite 

 may present peculiarities of composition. The facility with 

 which the mineral can be isolated from the Sopa schist gave an 

 opportunity to determine roughly, by washing, its proportion 

 as about 0*01 per cent. It is apparently much less abundant in 

 the Serra do Gigante rock and therefore could hardly be ex- 

 pected to appear in the quantity ordinarily employed in a 

 chemical analysis. It seems probable also that this rock carries 

 some other undetermined phosphate. 



Commissao Geographica e Geologica, 

 S. Paulo, Brazil. 



