866 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



for the chemical changes due to solution in weathering to be only incipient 

 until hydration and mechanical disintegration have become extensive.^^^ 



This mineral appears consequently to be well adapted for determining 

 the age of the formation, but in the analysis it is noted that the total is 

 but 96.50 per cent, indicating a loss of 3.5 per cent. This resulted from 

 the limited quantity of the sample and the presence of the rare earths 

 wliir-li required it to be divided into three portions. Hillebrand states 

 til at ^^it is probable that the loss should be distributed somewhat unevenly 

 over a number of the constituents, silica and lead excepted, they being 

 without doubt nearly correct." If 1 per cent of this loss should be added 

 to the 19.75 per cent of uranium, the mineral would possess 20.75 per cent 

 uranium and the lead-uranium ratio becomes .167, exactly the same as 

 for analysis number 15, in which the total amounts to 99.93. 



In regard to number 17, yttrocrasite. Hidden and Warren make the 

 following observations : 



"The crystal from M^hich the material for analysis was selected was found 

 in Burnet County, Texas, by Mr. John J. Barringer, who discovered the famous 

 gadolinite mine just across the Colorado River, in Llano County, now known 

 as Barringer Hill. 



"The crystal had a thin dull brown coating of amorphous material which 

 was evidently an hydrated alteration product, very similar to the yellowish 

 brown coating observed on the polycrase (?) of North and South Carolina. 

 The fresh underlying material is black in appearance and has a bright, pitchy 

 to resinous luster, and closely resembles that of polycrase and euxenite, and 

 like these has an uneven and small conchoidal fracture. Its hardness Is be- 

 tween 5.5 and 6. 



"The mineral when examined between crossed nicols is seen to consist of a 

 mixture of isotropic and a feebly double refracting material. In several in- 

 stances a distinctly spherulitic structure was observed with high powers ; other- 

 wise nothing of a definite nature could be made out regarding the optically 

 active portion. The mineral is not now, therefore, of a strictly homogeneous 

 structure. The fact, taken into consideration with its content of water and 

 carbon dioxide, 0.68 per cent, suggests that the mineral is a hydrated altera- 

 tion of an originally anhydrous species. It may be mentioned here that a very 

 similar heterogeneous structure has been also noticed as characteristic of speci- 

 mens of polycrase (?) from North and South Carolina." ^^* 



In this connection should be discussed the significance of the hydration. 

 If it is due to the leaching of descending ground waters it would indicate 

 a geologically recent alteration, which would raise doubt as to the value of 

 the lead-uranium ratio. The rather well crystallized nature of certain of 

 the uranium and thorium hydrates and the lack of accompanying oxida- 



123 G. P. MerriU : Rocks, rock weathering, and soils. 



124 Am. Jour. Sci., vol. 22, 1906, p. 515. 



