492 proceedings: geological society 



iron as the oxide, leaving silica and insoluble Mg and Al silicates. 

 In general, the percentage of quartz increases with increase of exposure 

 of the soil to the weathering agencies. 



Discussion: R. B. Sosman inquired whether any distinction is made 

 between the effects of purel}^ mechanical processes of destruction and 

 those due to chemical alteration. Fry replied that in soils there was 

 usually no means of finding out which process had been most effective. 

 D. F. MacDonald inquired whether sizing had been used in 

 the analysis of soils. Fry said that it had. F. E. Wright asked 

 as to percentage of quartz grains in soils, and also as to whether the 

 mineral composition gave any indication of fertility. Fry answered 

 that in the Norfolk soil the silica formed 97 per cent. As to fertility, 

 the mineral composition formed at least as good a basis for judgment 

 as chemical analysis. E. T. Wherry asked whether the secondary 

 enlargement of quartz grains took place after the grains had left the 

 parent rock. Fry said he had no means of deciding this. La Forge 

 asked regarding limestone soils — whether the variety of minerals 

 found could be attributed to contributions by wash from foreign 

 sources. Fry said that limestone soils showing no such contributions 

 possessed a great variety of minerals. 



G. R. Mansfield: Geology of the Fort Hall Indian Reservation, 

 Idaho. The physiographic history is complex, involving at least three 

 cycles of erosion. Numerous physiographic features are due to vul- 

 canism. The sedimentary rocks include a long sequence of formations 

 ranging in age from early Cambrian to Quaternary, but with no rep- 

 resentatives of the Cretaceous. The geologic section corresponds 

 with that of the Montpelier district in southeastern Idaho. The re- 

 vision of certain Triassic formations in the reservation is found advis- 

 able. The Nugget sandstone is divisible into four members: (4) 

 main sandstone member 1500 =t feet thick, (3) Wood shale 220-250 

 feet, (2) Deadman limestone 150 feet, (1) Higham grit 500 feet, at the 

 base of the formation. The Thaynes limestone becomes the Thaynes 

 group with three members: (3) Portneuf limestone at the top 1500=1= 

 feet, (2) Fort Hall formation 800 feet, (1) Ross hmestone 1350 feet. 

 Igneous rocks occur in considerable variety and abundance, with much 

 fragmental material. An interesting single occurrence of nepheline 

 basalt is reported. The general sequence of igneous rocks seems to 

 have been (1) an earlier basic or intermediate lava, (2) acid eruptives, 

 (3) basalt, (4) latite, the last being perhaps as late as middle or late 

 Pleistocene. The structure is very complex in detail and is marked 

 by both faulting and folding. There seem to have been at least three 

 epochs of deformation. The Putnam Overthrust, a fault comparable 

 with the Bannock Overthrust and with other great faults in the Rocky 

 Mountain region, is particularly noteworthy. Phosphate deposits 

 occur in Ihe eastern part of the reservation. The main bed appears 

 to average about 6 feet in thickness and to be of 70 per cent or 

 better quahty. The tonnage estimate for the reservation is 738,526,700 

 long tons. Other mineral deposits of the reservation are negligible 



