39662 str (73)

126 ROCKS

due to the fact that too smali a rangę of I studied. Leighton finds that an ice-cr_ea^reSses thixotropic. The “basie” (i.e., stirred) ^³*¹** is obeys Arrhenius’ logarithmic law. Batem^1SC°^sity Sharp find a similar thixotropy for milk, whi^ ^atl<i explain as being due to fat dumping. ’ p ^y

The rheology of rocks has an important beari on geology. Adams and Williamson have measu^ the compressibilities, the bulk and the rigidit moduli, and the densities of different kinds of rock at pressures varying from 2-10 Kg./cm.². ||k inereasing pressure the compressibilities fali and the moduli rise correspondingly. The density inereases with pressure. The structure of the earth itself has been studied from the rheological point of view, and in this connection Nadai says :—

i The compressive stresses inside the continents produced by the weight of the floating rock masses may, under the influence of various factors (variations in the cohesion or structure of the various sedimentary layers produced by previous geological processes, differences in the viscosity coefficient, action of plastic layers between rigid layers, transgressions of inland seas, with the accompanying deposition of less rigid sediments on strips of other morę rigid sediments, and other factors), be changed so that a sufficiently large difference has gradually formed between the pressures in vertical and horizontal directions to cause plastic distortion.”

It is claimed that the deposition of rock salt in big beds has come about on account of its greater plasticity than that of the surrounding rock.

Professor Mead, of Madison University, Wisconsin, has madę an interesting model to show the plastic behaviour of geological materials. A rectangular strip of rubber is attached by one side to a rigid frame, and the opposite side is fastened to a piece of

| _włu_Ch can be drawn backwards and forwards '^V°°means of a winch, so that the rubber can be ^tended or partially contracted in one piane. Clays, cands, etc. can be deformed by laying them on this sheet. 1 sulphur is mixed with the sands, the distorted błock can be transferred to an oven. Heat here melts the sulphur, which afterwards re-sets, forming a hard mass, which can be cut up to study the flow lines, etc.

The flow of alpine rocks has been demonstrated by the fact that the axes of smali ąuartz crystals, over an area of many square miles, all show a common direction. This is very reminiscent of the orientation of crystals in metals, following severe plastic deformation. Schmidt ąuotes an interesting case with gamets in shale. These are orientated at an angle to the planes of scaling of the shale. Originally the entire rock was sheared parallel to the piane of scaling. Later, the garnets were formed by crystal-lisation and, being much morę rigid than the shale, when further shear occurred they turned round, like bali bearings in a race.

BIBLIOGRAPHY

Adams and Williamson. J. Franklin Insł., 1923, CXCV., 475. Bateman and Sharp. /. Agric. Res., 1928, XXXVI., 647; J. Dairy Sci., 1928, XI., 380.

Broughton and Squires. J. Phys. Chem., 1938, XLII., 253. Carver and Folts. /. A mer. Chem. Soc., 1925, XLVIŁ, 1430. Einstein. Ann. der Phys., 1906, XIX., 289.

Eirich and Goldschmidt. Koli. Zeits., 1937, LXXXI., 7. Hatschek. " The Viscosity of Liąuids,” 1928. Bell. Houwink. 6ster. Chem. Zeitung., 1937, No. XXI.

Jong, Bungenberg de, and Sian Gwan. Bwchem. Zeits., 1930, CCXXI., 166, etc.

KdSTER. Die chemische Fabrik, 1936, IX., $81.

Leighton and Kurtz. J. Phys. Chem., 1929, XXXIII., 1489.