MAPPING OF THE CRYSTALLINE SCHISTS 859 



the science of geology, and that, whereas the mechanics of 

 crustal deformation, as now understood, and observation alike, 

 take account of two distinct methods of rock deformation, geo- 

 logical mappers seem generally to have regarded but one as 

 possessing much significance. The subject is an intricate one 

 and one which does not readily lend itself to absolute demon- 

 stration, but in another place concrete examples will be chosen 

 and described in some detail to show that the subject is a much 

 larger one than is ordinarily understood. 



DEFORMATION UNDER COMPRESSIVE STRESSES EITHER BY FLOW 



OR BY RUPTURE. 



The stresses within the earth's crust which bring about suffi- 

 ciently important deformation to affect the tectonic structure of 

 an area may be assumed to be vertical compression due to the 

 weight of superincumbent material, and lateral compression due 

 to crustal shortening from whatever cause, 1 but in the last 

 analysis a result of gravitation. Tensile stresses are locally pro- 

 duced when rocks assume new attitudes under compressive 

 stress, but their importance as a direct cause of deformation 

 over considerable areas is probably subordinate and largely 

 limited to the effect of temperature change in surface masses 

 of igneous rock, and hence may largely be dismissed from con- 

 sideration in the present discussion. 



The generally accepted classification of displacements result- 

 ing from earth stresses has been thus expressed by Suess : 2 



The visible displacements 3 in the earth's superstructure of rock are the 

 result of movements which proceed from the diminution of volume of our 



i Van Hise, "Estimates and Causes of Crustal Shortening," Jour. Geol., 

 Vol. VI (1898), pp. 10-64, also, "Earth Movements," Trans. Wis. Acad. Set., etc., 

 Vol. XI (1898), pp. 465-516. 



2 Suess, Das Antlitz der Erde, 1885, Vol. I, p. 143. See also, Margerie et 

 Heim, Les dislocations de V ecorce terrestre, Zurich, 1888, p. 8. 



3 The double meaning of the German and French words Dislocation and disloca- 

 tion is especially unfortunate. In continental usage " dislocation " is employed in its 

 more general sense for "displacement" (as for example, in the title of Margerie and 

 Heim's work), and quite generally also in a special sense for "disjointing," or equiv- 

 alent to "faulting." American usage is, I believe, wholly restricted to the word 

 applied in the latter sense. 



