310 0. A. Derby — Occurrence of Xenotime ^ etc. 



the xenotime occurs with an abundance of monazite in a thin 

 black totally decomposed f eld spathic layer intercalated in a 

 black schistose highly micaceous gneiss, which is poor in acces- 

 sories and, like most of the gneiss and granites in the district 

 (about Petropolis), affords only zircon. This difference and 

 the general character of the layer suggests a suspicion that, 

 instead of a segregation as was presumed without a very close 

 examination, it may prove to be a dyke of muscovite granite 

 modified by dynamo metamorphism. It is on the xenotime 

 crystals of this layer that diametric faces were observed. Two 

 of the Sac Paulo localities are of muscovite-biotite granites, 

 one of which forms a considerable boss erupted through Cam- 

 brian (?) schists. It is noticeable that these biotite bearing 

 rocks afford extremely abundant residues of monazite and 

 xenotime, while the zircon is unusually rare. A dyke of 

 lithia-mica granite with muscovite, in the vicinity of one of 

 these bosses, is free from phosphatic minerals, but contains rare 

 grains of cassiterite associated with zircon. 



The presence of this yttrium phosphate appears to be in some 

 way dependent on that of potash, since only in the somewhat 

 doubtful case of Petropolis has it been observed outside of the 

 potash-granites, although hundreds of residues have been exam- 

 ined. The same cannot be said of its almost inseparable com- 

 panion, the cerium phosphate (only three xenotime-bearing 

 residues failed to show monazite, and this, perhaps, owing to 

 deficiency in the quantity of rock washed), which has proved 

 to be very frequent, though less constant, in the biotite gneiss 

 and granites, rare in those containing amphibole. Magmas 

 rich in soda, represented by phonolites, nepheline and augite- 

 syenites, seem to be unfavorable to both minerals, as neither 

 have been found in the dozen or more residues examined. The 

 other companion, zircon, on the contrary, rarely fails to appear 

 in the crystalline rocks, except in the most accentuated basic 

 types — such as diabase, basalts, etc. It may be noted further 

 that in the rocks rich in potash, as indicated by the presence 

 of muscovite, the zircon and monazite in the residues from 

 decomposed material are almost always somewhat altered, being 

 corroded, whitened, and more or less opaque, whereas, in the 

 biotite rocks they rarely show the slightest signs of alteration. 

 The latter observation applies also to the zircons of the soda- 

 bearing rocks as far as these have been examined. 



The almost constant occurrence of the cerium and yttrium 

 phosphates, monazite and xenotime, in the Brazilian rocks sug- 

 gests the hypothesis that they will be found in similar rocks 

 all over the world. A recent visit to the United States has 

 enabled me to test this hypothesis on a small number of Xorth 

 American granites and gneisses. Drift bowlders picked up in 



