0. A. Derby — Monazite as an Element in Rocks. 113 



containing 45 holes to the linear inch. As the decayed feld- 

 spar and mica, which may be presumed to carry the rarer and 

 first formed minerals of this rock, went much finer than this, it 

 was assumed that all of these were set free. From 1906 

 grams of the ground rock 0*557 grams or 0*029 per cent of 

 residue consisting mainly of the yellow grains were obtained. 

 As the small quantity of zircon and ilmenite in this residue is, 

 probably, but little if any in excess of the loss in washing, the 

 proportion of the yellow mineral can be safely put down as 

 from '02-0*03 per cent of the entire mass of this rock. 



A red syenite from the Serra do Stauba in the province of 

 Bahia gave the yellow mineral in comparatively large grains, 

 but these were few in number in comparison with the zircon. 

 A mass of clay from the station of San Joas on the Sorocaba 

 railroad in the province of Sao Paulo which is presumed to rep- 

 resent the syenitic rock of the vicinity, but which may be from 

 gneiss, gave, with abundant zircons, a mineral giving the same 

 reactions as those from the other rocks but lighter in color and 

 duller in aspect than is usual. 



The basic eruptives thus far examined, representing diabase, 

 quartz-diorite mica-diorite and minette have afforded no trace 

 of the yellow mineral. 



It should be mentioned that in all these tests care has been 

 taken to select samples representing the principal mass of the 

 rock free from veins and mineral aggregates. In the course of 

 these investigations grains which appear to represent several 

 other rare minerals have been met with, but these have not 

 yet been fully examined. 



Since the above was written, a test has been made on a rock 

 richer in monazite than any hitherto examined. This is a 

 fine grained granitite exposed in a large dyke in the road from 

 Engenho Noro to Jacarepagua in the outskirts of Rio de 

 Janeiro. After thorough drying in the sun 3002 grams of 

 the clay resulting from the decomposition of the rock was 

 washed and the residue cleaned by the use of a heavy solution 

 (sp. gr. 3 "5), and of the electro-magnet. The residue weigh- 

 ing 2 "24: grams, or 0*0746 per cent of the entire mass, consists 

 principally of monazite in exceedingly fine grains with a 

 small amount of zircon and a much smaller amount of other 

 impurities that could not be completely separated without loss 

 of material. The mixed monazite and zircon can safely be 

 put down as 0*07 per cent of the rock. 



In a recent excursion to the Argentine Republic Mr. Gor- 

 don obtained residues of zircon and monazite from the river- 

 sands at Buenos Ayres and from gneiss and granite decom- 

 posed in situ at Cordoba. 



Am. Jour. Sci— Third Series, Vol. XXXVII, No. 218.— Feb., 1889. 



