SEPTEMBER, 1875. 



ACTION OF DENUDING AGENCIES. 



ABSTRACT AND CONTENTS OF LECTURE, &c. 



March 22, 1875. 

 By A. Tylor, F.G.S. 



PAGES 



1. Atmospheric conditions at the time of arrival of man upon the earth 



very different to present conditions, in fact a pluvial period... ... 437 



2. Method of formation of rock basins or deep lakes in mountainous 

 districts by excavation out of solid rocks by lake glaciers ... 438, 446 



3. New explanation of mode of excavation by glaciers, dependent or 

 consequent upon the expansion of water at the bottom of the lake 

 during the act of freezing ; this water not being produced by the sun's 

 heat, but by the friction of ice upon ice during the motion of the 

 glacier. Fig. 1 439 



4. Glacier motion described and compared to that produced by building 

 upon a marsh a tower or railway embankment ... ... ... 440 



5. Deep lakes are never found except close to mountains, as represented 



Fig. 2 ... ... ... 439,440 



6. Machine for proving the amount of friction of ice upon ice, or what is 

 termed the co-efficient of friction. Fig. 3 ... ... ... ... 441 



7. Experiment on velocity of sliding bodies, proving that velocity is 

 nearly twice as great when the weight on one of two similar sized 

 pieces of ice is increased eight times. Fig. 4... ... ... ... 442 



8. Regelation only occurs when there is a thin film of water between two 

 pieces of ice, that is, when the quantity of water is so small that part 

 can be evaporated or conveyed by endosmosis into the pores of the 

 adjacent ice as if it were a colloid, abstracting heat from the remaining 

 water ... ... ... ... ... ... ... ... 439,441 



9. Rivers. Absence of a special work on rivers remarkable, when there 

 are so many books on glaciers. It is important to know the quantity 

 of siliceous matter rivers push annually out to sea, as this is probably 

 twice as large as the material they carry off the land in suspension. 

 I estimated in 1853 that the whole land of the globe was lowered one 

 foot in nine thousand years by river action ; but by conputing the 

 siliceous matter pushed out to sea, I correct the calculation, and esti- 

 mate the average denudation at one foot in two thousand years ...443, 469 



10. Explanation of uniform mean motion. Proof by observation that the 

 mean velocity of every navigable river is nearly constant throughout its 

 course, on any day, or that increase of quantity flowing has an equal 

 effect in increasing velocity that decrease of slope has in decreasing 

 velocity : Fig. 9. The equation to uniform motion is given page 447, 466 



11. Longitudinal section of Rhine, showing that hard strata raise the flood 

 level above the theoretical curve, and that soft strata allow the flood 



DECADE II. — VOL. II. — NO. IX. 28 



