492 T. L. WATSON WEATHERING OF ALLANITE 



the important chemical reactions in the decomposition of the mineral, for 

 in the weathering of lime and magnesia-bearing rocks these constituents 

 invariably suffer loss from leaching in the form of soluble carbonates 

 (bicarbonates). The percentage loss of lime and magnesia certainly, and 

 of the cerium earths probabl}^, indicated in the table above, is due to the 

 formation of soluble carbonates and their removal in solution. 



Simultaneously with the production of carbonates, most of the silica 

 separated as colloidal silicic acid, which is readily soluble, and was re- 

 moved in solution,^^ which accounts for the unusually large percentage 

 loss in this constituent. Alumina has likewise been removed in larger 

 quantity than usual in weathering, although several cases have been 

 recorded among siliceous crystalline rocks in which the loss in this con- 

 stituent equals that in the case here described. 



Special attention is directed to the large percentage loss of the cerium 

 earths, the figures indicatirrg that all of the lanthana, 96.66 per cent of 

 the didymia, and 88.28 per cent of the ceria have been removed by solu- 

 tion. These figures should be compared with those for the Amherst and 

 Nelson counties localities. 



The decomposition of the mineral has been accompanied by a total loss 

 from leaching of 72.53 per cent, or nearly three-fourths of the original 

 material. Of the original silica, 93.23 per cent has been lost, 93.02 per 

 cent of alumina, and 36.48 per cent of the titania, while all of the iron 

 oxide has been retained. All of the magnesia and alkalies and 94.38 per 

 cent of the lime have disappeared. 



NELSON COUNTY 



Locality and mode of occurrence. — The allanite of Nelson County is 

 found about 3 miles east of LowesviDe. It has been described by C. G. 

 Memminger,^^ who analyzed it with the results shown in column I of the 

 table below. The allanite is reported to occur in a vein at a depth of 

 4 or 5 feet below the surface in the form of detached masses imbedded 

 in a ^^clay" derived from its weathering. The masses of mineral were of 

 rounded outline and covered with a thin yellowish white crust formed by 

 its weathering. 



Fresh allanite. — The general properties of the allanite from the Nelson 

 County locality are as follows: Color, pitch black; luster, resinous; frac- 

 ture, uneven, with traces of cleavage ; streak, greenish gray ; hardness, 6 ; 

 and specific gravity, 3.59. It fuses before the blow-pipe with intumes- 



■^0 L. Kahlenberg and A, T. Lincoln : Jour. Phys. Chem,, vol. 2, 1898, pp. 77-90. 



C. R. Van Hise : Monograph xlii, U. S. Geol. Survey, 1904. 

 80 C. G. Memminger : Am. Chem. .Tour., vol. vii, 1885-1886, p. 177. 



