PART I. • CHAPTER VI. 



79 



Origin of Great Faults Denudation of Rocks. 



terranean cavity ; but it is far more probable that the sinking 

 will be effected at successive periods during different earthquakes, 

 the mass always continuing to slide in the same direction along 

 the planes of the fissures f g, and the edges of the falling mass 

 being continually more broken and triturated at each convulsion. 

 If, as is not improbable, the circumstances which have caused 

 the failure of support continue in operation, it may happen that 

 when the mass B has filled the cavity first formed, its foundations 

 will again give way under it, so that it will fall again in the same 

 direction. But, if the direction should change, the fact could not 

 be discovered by observing the slickensides, because the last scor- 

 ing would efface the lines of previous friction. In the present 

 state of our ignorance of the causes of subsidence, an hypothesis 

 which can explain the great amount of displacement in some 

 faults, on sound mechanical principles, by. a succession of move- 

 ments, is far preferable to any theory which assumes each fault 

 to have been accomplished by a single upcast or downthrow of 

 several thousand feet. For we know that there are operations 

 now in progress, at great depths in the interior of the earth, by 

 which both large and small tracts of ground are made to rise 

 above and sink below their former level, some slowly and insen- 

 sibly, others suddenly and by starts, a few feet or yards at a 

 time ; whereas there are no grounds for believing that, during 

 the last 3000 years at least, any regions have been either up- 

 heaved or depressed, at a single stroke, to the amount of several 

 hundred, much less several thousand feet. 



CHAPTER VI. 



DENUDATION, AND THE PRODUCTION OP ALLUVIUM. 



Denudation defined — Its amount equal to the entire mass of stratified deposits 

 in the earth's crust — Horizontal sandstone denuded in Ross-shire — Level surface 

 of countries in which great faults occur— Connexion of denudation and alluvial 

 formations — Alluvium, how distinguished from rocks in situ — Ancient aUuviums 

 called diluvium— Origin of these— Erratic blocks and accompanying gravel — 

 Theory of their transportation by ice. 



Before we take leave of the aqueous or fossiliferous rocks, 

 we have still to consider the alluvial formations. Denudation, 

 which has been occasionally spoken of in the preceding chapters, 

 is the removal of mineral matter by running water, whether by 



