294 proceedings: geological society 



measuring the quantity of surface water that seeps into the earth and 

 percolates into the zone of saturation; the second in measuring the 

 ground water that is discharged through springs, or by evaporation 

 from soil and plants ; the third in observing the fluctuations in the water 

 table, which represents filling or emptying of the ground-water reser- 

 voir; the fourth, like the gaging of surface streams, in measuring the 

 flow of ground water at selected cross-sections. 



Discharge methods comprise two very different kinds of methods: 

 (i) the difl'erentiation of ground-water run-off from direct run-off 

 through study of hydrographs, etc., and (2) the mapping of areas that 

 discharge ground water by evaporation and transpiration, and experi- 

 ments to determine rates of discharge under specific conditions of soil, 

 vegetation and depth to water table. In arid regions plants of certain 

 species habitually utilize water from the zone of saturation. For such 

 plants the name phreatophyte, meaning a "well plant," has been pro- 

 posed. 



The water-table methods are best adapted to regions, such as Cal- 

 ifornia, which have well-defined rainy and dry seasons. The average 

 annual increment to the ground-water supply can be computed by 

 multiplying the average rise of the water table in the rainy season by 

 the percentage of available pore space, and multiplying this product 

 by the area of the water table of the given aquifer. The most uncer- 

 tain factor is the percentage of available pore space, or specific yield, 

 which is not the same as the porosity of the rock, because when the 

 water table descends some of the water is held against the pull of gravity 

 by the molecular attraction of the rock. Tests of specific yield have 

 been made (i) by laboratory experiments; (2) by examination of sam- 

 ples of material which is above the water table but which in the high- 

 water stage was in the zone of saturation; and (3) by making hea\y 

 pumping tests and ascertaining the total pumpage and the total volume 

 of sediments drained thereby. 



The two lines along which additional research are most needed 

 relate to the habits of phreatophytes and their rates of transpiration 

 and to the specific yields of different kinds of sediments. A very for- 

 tunate feature of the quantitative work is that the three principal 

 methods — intake, discharge, and water-table — are entirely independent 

 of each other and can be used as checks upon one another. 



The 333rd meeting of the Society was held at the Cosmos Club on 

 Wednesday evening, March 26, 1919, President Ulrich presiding, and 

 48 persons present. 



INFORMAL communications 



F. L. Hess: Phenocrysis in granitic intrusions. Deposits of rare 

 metals are commonly connected with granites and are usually in the 

 outer parts of granitic intrusions, and it thus happens that many of the 

 deposits are accompanied by extraordinary forms of the granite. Close 

 to the cobalt deposits on Blackbird Creek in Lemhi County, Idaho, 

 a remarkable porphyritic granite is found on Big and Napias Creeks. The 



