468 SCIENCE PROGRESS. 



mountain masses are moving in opposite directions, and 

 generally, recognisable fragments of the neighbouring rock 

 are disseminated throughout the breccia ; in some cases there 

 is gradual transition from the one to the other, whereas in 

 others, the boundary is sharp, or the breccia may be intruded 

 into the parent rock in a series of tongue-like projections. 



(7) The overthrusts often follow but one fault plane, 

 though more commonly they have taken place along several, 

 giving rise to a step-like formation (the Schuppen structure 

 of Suess). It is possible, therefore, for a single overthrust 

 to be broken up into a number of small ones, and vice versa. 

 We have also examples wherein the greater cut off the less, 

 the former being known in the Scotch survey as major, and 

 the latter as minor overthrusts. 



(8) Though it apparently holds true that in every 

 mountain chain overthrusts have occurred, in many cases 

 they have not been abundant, their place being then taken 

 by vertical faulting, due to the elevation of one side of the 

 fault, this being the result of the same act of compression 

 which produces the phenomena of overthrust ; this condi- 

 tion is conceivable if movement coming from one side only 

 should have resulted from the compression of the deeper 

 earth-mass. 



(9) It is difficult, however, under these circumstances to 

 sub-divide faultings into (1) heaves, in which, by the means 

 of pure tangential forces, horizontal movement alone with- 

 out vertical displacement has been produced ; (2) faults, 

 due entirely to the action of gravity ; and (3) overthrusts, 

 clue to a combination of tangential and vertical forces. In 

 faults having a hade, the direction of the movement may be 

 easily traced ; but in such as are perpendicular it is difficult 

 to determine whether the present position of the lower-lying 

 beds has been reached through a sinking on their side of 

 the fault, or is due to elevation on the other. Thus, in the 

 Eastern Alps, troughs have been broken up in such a 

 manner that the base appears to overtop higher portions of 

 the trough limb, and it is uncertain whether it has risen, 

 or the other sunk with regard to it (see fig. 106, Geo- 

 Tecktonische Problcme). Similarly, the central part of an 



