DYNAMICAL GEOLOGY. 2O/ 



material as it rubs against the floor and sides of stream and 

 river-channels (erosion), and in the accumulation of the trans- 

 ported material as sediments (deposition). The strength of 

 the processes of transportation and erosion depends on the 

 volume and velocity, or the impulse, of the running water. 

 The transportation power of streams and rivers is under 

 ordinary circumstances confined within their channels, but 

 although of limited extent it is a phenomenon apparent to 

 every observer because of the energy of motion displayed. 

 The washing away of rock-material by rain is much less 

 apparent, but it is extended over far vaster tracts of country. 



A great incentive was given to the scientific study of surface- 

 forms and their causes by the brilliant work of the American 

 investigators, Hayden, Powell, Gilbert, Button, and others. 

 While they described the wonderful river erosion that had 

 taken place in the table-lands of the western states, European 

 travellers were making known the characteristic forms of 

 erosion in the high and barren territories of inland Africa and 

 Asia. There the irregularities of the surface are chiefly due to 

 the periodic occurrence of torrential rains and the consequent 

 sudden increase or rapid rise of mountain-streams, which rush 

 as destructive floods over the table-lands, and retreat and 

 diminish no less rapidly than they arose. 



Earth-pillars or pyramids occur in majestic forms in some 

 places, and offer more familiar examples of the surface-waste 

 accomplished chiefly by rain. In miniature, the formation of 

 an earth-pillar may be observed in any thick foliage wood after 

 a heavy shower of rain. The drops as they fall from the 

 leaves upon the soil sometimes alight upon small pebbles, 

 sometimes upon soft humus. The latter is readily washed 

 away, the pebbles remain and serve as protecting caps to the 

 soil immediately below, so that each pebble and the under- 

 lying soil gradually stands out as an individual column. Rain- 

 eroded pillars occurring on a grand scale in the Hautes Alpes 

 were described in 1841 by Surell; Sir Charles Lyell described 

 pillars in the morainic conglomerate in the Tyrol, where the 

 larger boulders had served as capping-stones. Hayden made 

 known magnificent examples in the conglomerate rock of 

 Colorado. Sir Archibald Geikie described their occurrence at 

 Fochabers, in the north-east of Scotland, in Old Red con- 

 glomerates. 



Although many writers of the eighteenth century had devoted 



