proceedings: geological society 491 



of these is due to contraction in a physically heterogeneous material 

 (mixture of solid and liquid) and is fundamentally different from the 

 type produced by prismatic division in a cooling solid; a common 

 example is mud cracks produced by drying. The second type is due 

 to internal expansion, whereby the surface is stretched and broken; 

 a prismatic structure produced in a cement briquet by internal ex- 

 pansion was shown as an example. It was suggested that the "weather 

 cracks" on the surface of diabase boulders are of this type, and that 

 they are due to sub-surface hydration and expansion, which produces 

 a tension in the surface of the block. 



REGULAR PROGRAM 



R. C. Wells: The solubility of magnesium carbonate in natural 

 waters. Under atmospheric conditions at 20°C. magnesite was found 

 to dissolve in pure water to the extent of 0.018 gram magnesium and 

 0.065 gram total carbon dioxide per liter, and somewhat more in solu- 

 tions of other salts. But some natural waters freely exposed to the air 

 contain much more magnesium and carbon dioxide than this. A true 

 equilibrium was obtained at 20° only with MgC03.3H02 as solid phase, 

 the final solubility being 0.37 gram magnesium and about 1.00 gram CO2 

 per liter. 



Discussion: T. Wayland Vaughan said he had been greatly inter- 

 ested in a closely related subject — that of the solubility of calcium 

 carbonate in sea-water. He had come to the conclusion that the sea- 

 water was very nearly saturated with calcium carbonate and that 

 anything which disturbed the equilibrium would be apt to precipitate 

 the carbonate. He had experiments in mind to ascertain whether 

 MgCOs might not be thrown down with CaCOs under natural conditions. 



W. H. Fry: The 'weathering stability of minerals as illustrated in soils 

 and soil-like materials. Soils from various climatic, physiographic, and 

 geologic regions of the United States were examined petrographicallj'. 

 The minerals identified are as follows: quartz, orthoclase, plagioclase, 

 muscovite, biotite, hornblende, augite, calcite, dolomite, chlorite, ser- 

 pentine, olivine, kaolin, sericite, limonite, hematite, magnetite, and a 

 great variety of less common minerals. As to mineralogical composi- 

 tion, the various soils resemble each other qualitatively; but quantita- 

 tively they differ widely. Orthoclase occurs both fresh and altered. 

 Microcline always occurs as fresh fragments. Acid plagioclases gener- 

 ally appear as fresh grains, while the more basic ones are at times deeply 

 altered. Hornblende sometimes shows a tendency to alter to chlorite. 

 Epidote shows practically no signs of chemical alteration. The micas 

 are apparently very stable. Tourmaline, rutile, and zircon usually 

 occur as fresh crystals, although occasionally the edges are rounded. 

 Garnet appears to be fairly resistant. Magnetite appears to alter to 

 the hydrated sesquioxide of iron. Quartz is the most abundant of 

 the soil minerals, and occurs both as primary and secondary grains. 

 Judging from the mineralogical composition, the processes of soil- 

 weathering tend to leach out the alkalies and alkaline earths, to separate 



