476 SCIENCE PROGRESS 



whatever the prevailing conditions. Prof. W.Salomon ("Die 

 Definitionen von Grauwacke, Arkose, und Ton," Geol. Rund- 

 schau, 191 5, 6, 398) regards arkose and greywacke as analogous 

 deposits derived respectively from a granitic or gneissic terrane, 

 or from an area of clay-slates, sandstones, and miscellaneous 

 rocks. It is essential that the deposits should have suffered 

 very little transport, and hence should consist of coarse, angular, 

 unassorted material. 



The origin of dolomite has been investigated by F. M. 

 Van Tuyl (Amer. Journ. Sci. 191 6, 42, 249), who finds that 

 most of the dolomites examined are replacement products of 

 limestone. The facts supporting this contention are the 

 lateral gradation of beds of dolomite into limestone, the 

 mottling of limestone by irregular patches of dolomite on the 

 borders of dolomite masses, the existence of remnants of 

 unaltered limestone in dolomite, the irregular boundaries 

 between beds of dolomite and limestone, the presence of altered 

 fossils, the observed protective effect of shale beds, and the 

 general obliteration of structures and textures. 



Alteration of Rocks and Minerals. — An unusually perfect 

 example of pyrophyllitisation, pinitisation, and silicification 

 is described by A. F. Buddington (Journ. Geol. 191 6, 24, 130) 

 in the rhyolites and basalts of Conception Bay, Newfoundland. 

 The alteration is ascribed to metasomatic replacement ol 

 previously silicified rhyolites by thermal waters under con- 

 ditions of dynamic stress, and takes place in fault and sheai 

 zones. 



Important experimental work on the hydrothermal altera- 

 tion of felspars and hornblende has been performed by E. A. 

 Stephenson (Journ. Geol. 191 6, 24, 180). Dilute solutions oi 

 sodium carbonate, potassium fluoride, mixtures of these two, 

 sodium bicarbonate, and sodium tetraborate were used on 

 crystals of adularia, albite, and hornblende. The reactions 

 were carried out in copper tubes heated in a special electric 

 oven at temperatures up to 280 C, and at pressures up 

 to 65 atmospheres, for periods from 15 to 90 days. Pure 

 water under these conditions had no effect whatever on the 

 minerals, but the alkaline solutions attacked the felspars witl 

 separation of silica, and generally with the formation of minut< 

 icositetrahedra of analcite. This has an important bearing 

 on the question of the origin of analcite and other zeolites in 



