436 Abstract of Lecture. 



PAGES 



stability. Directly vegetation covers the slope the surface becomes per- 

 manent. The great floods in France have been promoted by the woods 

 being cut down. The roots would have retained the soil and the 

 rainfall. The channels being choked by soil washed in, the cross 

 sections are reduced, and enormous floods are the consequence. The 

 process of denudation by water passing through the permeable bed 

 (A, Fig. 23, page 26, and a, Fig. 24) eventually causes the destruction 

 of the strata and the cutting back the valley by the stream. Also see 

 Fig. 28, Black-Gang Chine 455>45 6 



26. Denudation of ths Weald and formation of its river gorges. The 

 process of gorge formation is illustrated by Fig. 25, the current being 

 directed in a particular direction by unequal elevation of the strata. 

 Arthur Severn, the well-known artist, has embodied the theoretical 

 view of Fig. 25, in two excellent views, Fig. 27 and 28, representing 



the course of the denudation of the Weald gorges ... ... ... 457 



27. The consequence of unequal rainfall at different times of the year is 

 illustrated in Fig. 31 from observation, causing a constant change of 

 bed of rivers and horse-shoe bends, deposit occurring on the shallow 

 side of a stream, and excavation on the deep side ... ... ... 459 



28. Representation of a gorge cut out in soft strata at Black Gang Chine 

 by a stream — a type of the denudation produced in hard strata during 

 the pluvial period. Fig. 29 . All contours of ground are really suc- 

 cessions of walls and slopes ... ... ... ... ... 460, 465 



29. A gorge cut in limestone, probably bored first by an underground 

 river at Dovedale, Derbyshire, forming successive walls and slopes ... 461 



30. The section across a series of parallel valleys cut down to an imper- 

 meable bed is illustrated by Fig. 35, page 32. It is evident that now 

 rainfall, or pluvial or fluvial denudation, will not much affect the level of 

 the watershed or of the rivers. All the denudation will occur at the level 

 where springs escape, and all the valleys will be widened but not 

 be deepened, and the main level of the land above the sea will not be 

 affected to any great extent even by a great denudation. The drawing 

 represents a section through the Wealden Valleys ; the position of the 

 Weald or gait clay at unequal heights above the sea, east and west, 

 determines the course of the rivers, combined with the north and south 

 slope of the strata. The effect of a transverse, or east and west flexure 

 of 20 feet in a mile, modifies the effect of a north and south flexure of 

 the gradient of 1,000 feet in a mile, as at Guildford ... ... ... 462 



31. The fact of the exaggerated notion of the weight of the sun, for which 

 there is no careful calculation hitherto, is mentioned, Fig. 32, on 

 page 33. Pluvial period due to sun's influence ... ... ... 463 



32. The difference between an ellipse, Fig. 36, and an orbital curve, Fig. 



37, is explained on pages ... ... ... ... ... 467,468,469 



33. Important element in calculating the denudation of any particular area 

 by the amount of material carried away by rivers, has been hitherto 

 omitted. The sand moved out into the sea beyond the mouth of the 

 river, and carried along the coasts, appears to be twice as much, at a 

 moderate calculation as the material removed in suspension. My 

 estimate of present mean denudation all over the world is one foot in 

 2,000 years... ... ... ... ... ... ... ... ... 469 



34. New view of the action of the barometer as indicative of the variation 

 of the motion of the atmosphere, and not mere weight of the atmo- 

 spheric columns. The effect of rain on the barometer is only the record 

 of motion in the atmosphere. Motion always affects pressure 



35. List of scientific papers, by A. Tylor ... ... 470 



