AGE OF THE LLANO SERIES 867 



tion suggest, however, that the hydration was accomplished in a decadent 

 stage of the pegmatitic crystallization. Such deep-seated hydration by 

 asce7iding hydrothermal waters tends to be pervasive, owing to the high 

 internal pressures and temperatures of the solutions, favoring diffusion. 

 Hydration due to the descending waters connected with weathering is, on 

 the contrary, characteristically not pervasive until admission to all parts 

 of a rock is prepared by an intimate cra/akling. ^ow a hydration dating 

 back to a decadent stage of the pegmatization might change the content 

 of uranium at that time, but would not affect the present lead-uraiiium 

 ratio. Iji view of tliese and other considerations, it vvouUl appear that 

 Holmes has been misled in speaking of the altered character of these 

 minerals as eloquent of their uselessness as an index of geological time. 

 The material on the market has all come from shallow pits, weathering 

 has altered the surface, but this recent action is distinct from what appear 

 to be primordial alterations. Furthermore, these occurrences fortunately 

 are within the inner valley of the Colorado Eiver, just above the flood - 

 plain level, are 500 feet below the old upland of the surrounding country, 

 and have consequently been exposed to erosion during only the last cycle 

 of valley-cutting. It is found in general that, in rock outcrops exposed 

 only to this last cycle of erosion, comparatively little chemical alteration 

 of the bedrock has been accomplished by weathering. Under the higher 

 uplands ground water has been active for very much longer periods of 

 time, but in the bottoms of the inner valleys, on the contrary, the I'ocks 

 are much fresher closer to the surface, indicating that the valley erosioTi 

 has been more by mechanical breakage accompanied by superficial altera- 

 tion. 



Although in this yttrocrasite the uranium is small in amount, yet so 

 far as the analytical errors are concerned the analysis is of a high order. 

 Boltwood states: 



"The writer (Boltwood) had the good fortune to meet Professor Warren at 

 the time this analysis was in progress and the latter kindly consented to talce 

 especial precautions in the determination of the lead and uranium." ^-' 



Experience seems to show, according to Holmes, that thorium minerals 

 give unreliable indices of age because some original lead is apt to be found 

 in such minerals with the lead originating from uranium. The degree 

 of admixture of original lead doubtless varies with the region and also 

 the mineral. Its quantity may be estimated by a careful determination 

 of the atomic weights, since the original lead appears to be mostly not 

 derived from the uraninm series. The nraninite of the Llano rec^ion is 



i2GAin. .Tour. Sci., vol. 2?,, 1907, p. 82. 



