The geothermic gradient may be taken as 3° C for every i oo metres of covering strata. Consider a series of strata accumulated to a depth of 13,00o metres ; the temperature at the base would approximate and the pressure 3,500 atmospheres. Some geologists, as Milch, Dawson, Daly and others, have attributed great powers of metamorphism to geothermal agencies of this kind and the common parallelism between stratification and schistosity seen in some terranes is attributed to superincumbent load. Oceanic salt deposits undoubtedly have suffered profound metamorphism in this way, for they contain hydrated minerals which are stable only over a narrow temperature and pressure interval. For silicate rocks, however, a very great cover of sedi ment would be necessary to induce reaction and recrystallization, principally for the reason that such reactions proceed with great tardiness at these temperatures and pressures ; indeed, clear evi dence that deep burial is insufficient to produce notable meta morphic effects in detrital sediments is provided in the great Palaeozoic geosynclines where the underlying strata can be studied in an almost unaltered condition.
The metamorphism of salt deposits is beyond the scope of this article, but reference may be made to E. Janecke, Die Entstehung der deutschen Kalisalzlager (1915), and F. Rinne, Die geo thermische Metamorphosen and die Dislokationen der deutschen Kalisalzlagerstiitten. Fortschritte der Mineralogie . . . vol. vi. (1920), where the subject is discussed at length.
Metamorphic rocks of this type constitute the greater part of continental shields (Fennoscandia, North America, West Australia, etc.). Where such rocks occur there is generally much evidence of earth movement, accompanied by crushing and folding. They are very characteristic of the central axes of great mountain chains, especially when these have been denuded and their deeper cores exposed. Most geologists believe that this connection is causal, holding that the contraction of the outer layers of the earth's crust, due to shrinkage of the outer shell upon a cooling and con tracting interior, has bent and folded the rocks, and at the same time has crushed and largely recrystallized them.
In extreme cases where dynamic action is localized, the ultimate stage of intense crushing and pulverization is the production of flinty crush rocks or pseudotachylytes, in which mylonization is accompanied by an incipient fusion of the rock mass, due to the generation of great heat by friction. Analogous effects are sometimes obtained in rock drills, where the cores are found to contain slag-like masses, produced by the heat of friction. In these cases it is shown that the temperature attained may approximate r,Ioo° C, sufficient to fuse arkoses or felspathic sandstones. Flinty crush rocks have now been recognized from a number of areas from the north-west Highlands of Scotland, the Outer Hebrides, and the Vredefort area of the Southern Transvaal. The trap shot
ten gneisses of southern India are similar rocks. In Eriskay and South Uist (Outer Hebrides), the flinty crush rocks are extensively developed along great thrust planes in the Archaean gneiss, while in the Vredefort area rocks of this character occur on a grander scale than elsewhere.
In areas of regional metamorphism, whole complexes consisting of intermixed sediments, tuffs and igneous extrusions and intru sions may be converted into a great series of schists and gneisses. Although recrystallization may be complete the original masses still retain their identity in their new state, though their struc tural relations may be rendered excessively complex by intense folding and overthrusting. The effects of stress in regionally meta morphosed rocks are revealed in their foliated or schistose textures. This consists in a definite arrangement of the minerals, so that such as are platy, prismatic or fibrous (e.g., mica hornblende or sillimanite) have their longest axes arranged parallel to one an other. For that reason many of these rocks split readily in one direction. Contortion or crumpling of the foliation is by no means uncommon, and the splitting faces are then undulose or puckered. Recrystallization under the influence of stress gives rise to the very characteristic crystallization-schistosity. On solid minerals at low temperatures the effects of stress are seen in crushing and shearing, and the development of glide planes or secondary twin ning (as seen in calcite, diallage, kyanite, etc.), but at higher tem peratures in the presence of interstitial liquid such as pervades all rocks, the most important effect is the raising of the solubility of the material stressed. Wherever an appropriate stress is reached, material is dissolved at the points of greatest stress, and rede posited at points where the stress is less. This principle (some times referred to as Riecke's principle) is obviously of funda mental importance in explaining the foliated textures of crystalline schists. Not only so, the material dissolved is capable of reaction and may be redeposited as new minerals. Stress thus facilitates chemical reactions and may be considered as a kind of catalyst, promoting reactions but also having influence on the nature of the products formed. Stress may thus influence fundamentally the stability range of a given compound ; on the one hand, the range may be extended or contracted ; on the other, reactions which can proceed only very slowly under uniform pressure may be greatly facilitated under stress. The possibility that thereby new phases may be precipitated in stressed systems while others are totally excluded is now to be envisaged. It is consequently not surprising that we find among the minerals of crystalline schists types which are unknown amongst contact rocks or are not synthesized in ordinary melts at uniform pressure. Minerals such as kyanite, chloritoid, lawsonite, glaucophane and staurolite are almost wholly restricted to stressed rocks, and are conveniently styled stress minerals. On the other hand, minerals such as orthoclase anorthite, andalusite and olivine are restricted to systems formed under uniform pressures and are referred to as anti-stress minerals. In contradistinction to thermal metamorphism, where many of the minerals are characteristically anhydrous, the lower grades of regional metamorphism are characterized by the hydroxyl-con taining minerals like sericite, chlorite, some amphiboles, and the epidote-zoisite group.