112 O. A. Derby — Monazite as an Element in Rocks. 



ent. Rutile appears to be a comparatively rare element in 

 these gneisses since the transparent red titaniferous grains re- 

 ferred to it were fonnd only in two or three places in peculiar 

 highly micaceous schistose layers, unusually rich in irou min- 

 erals. If, as is possible, these grains belong to some other 

 mineral, then rutile is entirely lacking in the rocks examined. 

 The gneiss from Socego and Cutia contains an abundance of 

 sillimanite. All the gneisses examined belong to the class of 

 biotite gneiss, except that from Santos which contains both 

 muscovite and biotite and in this the yellow grains are rare in 

 comparison with the zircon. No opportunity for examining 

 a purely muscovite gneiss has yet been afforded. The relative 

 proportions of zircon and the yellow mineral vary considera- 

 bly in these tests, sometimes the one sometimes the other pre- 

 dominating. In the rock from Socego and from Kil. 78 D. 

 Pedro II. railroad, the yellow mineral is particularly abundant. 

 A small number of granites have been examined with a 

 similar result, that is to say, all of them give zircon with a 

 heavy phosphate which in most cases appears to be identical 

 with the Bahia monazite. The greater number of tests have 

 been made on finegrained biotite granites which give residues 

 identical in appearance with those from the gneiss. The two 

 specimens of muscovite granite examined from the station of 

 Caieiras on the Sao Paulo railroad and from Sorocaba in the 

 province of Sao Paulo gave a small quantity of lusterless 

 whitish grains, quite different in appearance from those which 

 we had become accustomed to refer to monazite, but on sub- 

 jecting them to microchemical tests these also proved to be 

 cerium-bearing phosphates. Yellow grains of the ordinary 

 aspect are quite abundant in the small dykes of biotite 

 granite in the gneiss about Rio and also in the larger masses 

 of the Serra de Ti juca near Rio and at Pridade in the province 

 of Sao Paulo, where Mr. Henry Bauer has kindly made a test 

 for me. It is also abundant in uncommonly brilliant and per- 

 fect crystals in a small dyke in the gneiss of the Serra de Tin- 

 gua, a peak of the Serra do Mar range near Pio. They are 

 rare, in comparison with the zircon, in the large dykes near 

 Campo Grande on the Santa Cruz branch of the Dom Pedro II. 

 railroad, and near Bassa do Pirahy on the main line of the 

 same roads, and in a small dyke at a place called Boa Yista on 

 the Ribeira river in the Iguape region. It is interesting to 

 note that the first two of these rocks carry cerium as a silicate 

 in the form of orthite. The Tijuca granite is one of the rich- 

 est rocks yet examined in the yellow mineral and a rough 

 quantitative test was made on it as follows : A quantity of 

 the rock disintegrated but not completely decomposed was 

 dried in the sun and ground in a mortar to pass through a sieve 



