TRANSACTIONS OF SECTION C. 615 



some special case of (3), sediment is deposited comparatively near tlie shores of 

 continents. Even in the case of very large rivers only the finer sediment is carried 

 far from land. The Challenger soundings have shown that 150 miles is about the 

 maximum distance from land within which any important quantity of detrital 

 materials is deposited.^ As a rule (so far as I can ascertain), the coarser sediments 

 are generally deposited within a few miles of the coast. Hence this is fringed by a 

 zone of sediment, which, after passing a maximum thickness within a short distance 

 from the shore, gradually thins away. I doubt whether this detrital ft-inge is often 

 more than 70 or 80 miles wide : probably the coarser sands do not usually extend 

 for so much as a quarter of this distance. The inertia of the mass of the ocean 

 water quickly arrests the flow of even the mightiest river, or reduces it to a mere 

 superficial current. Hence the great ocean basins are regions where rock-building 

 is carried on slowly and chiefly by organic agency. Their borders bear the 

 burden, and the load taken off the continent is laid down on the bed of the 

 adjacent sea. 



(5) Thus rain and rivers are generally more important agents of denudation 

 and transportation than the sea, because unless the land be rising or sinking the 

 zone over which the latter can operate is limited both vertically and horizontally. 



(6) The coarser materials of rock are capable of being transported by streams to 

 considerable distances, without serious diminution of volume. Professor Daubr(5e has 

 proved experimentally that a stream flowing at the rate of about two miles per hour 

 would roll angular fragments of quartz or hard granite into perfectly smooth 

 pebbles after a transit of 25 kilometres (15f miles). During this process the frag- 

 ments lost about four-tenths of their weight. Further transport reduced the volume 

 of the pebbles very slowly. The loss afterwards varied from ~j^ to —--^ per kilo- 

 metre. To reduce a pebble of 2 inches diameter to 1 inch diameter — that is, to 

 diminish its volume by seven-eighths — would require a journey of from about 219 to 

 875 kilometres (approximately from 1-37 to 548 miles). This approximation, rough as 

 it is, becomes still less exact as the pebbles decrease in size ; the rate of diminution 

 in volume (cceteris j^nribus) bearing a relation to the area of the surface. Thus the 

 smaller the pebble, the further it will travel without material diminution of size. 

 Sand grains are even rounded with extreme slowness. According to the same 

 author a quartz grain i inch in diameter requires to be transported by water action 

 some 3,000 miles before losing its angles. On this accoimt the presence in a sand- 

 stone of numerous well-rounded grains is taken to indicate the action of wind, for, 

 as is well known, blown sands are much more quickly rounded.^ 



(7) Thus deposits of gravel and coarse sand, of considerable vertical thickness 

 and great extension, are more likely to indicate the immediate action of a river than 

 of a marine current. If limited in extent they may have been formed at the 

 embouchure of a river into a lake or sea. If, however, they can be traced for long 

 distances, they are more probably in the main sub-aerial deposits from rivers. 



The following examples may convey some idea of the kind of river which would 

 be required to transport the more important deposits of grits and stones mentioned 

 in the first section of this address : — 



The old river gravels of the Sierra Nevada are ' in some places 300 or 400 feet 

 thick and almost homogeneous from top to bottom,' sometimes they even obtain a 

 thickness of 600 feet. Mr. Whitney is of opinion that the fall in these old river 

 channels was probably from 100 to 130 feet per mile. Apparently, however, we 

 need not invoke so large a fall as this. The total fall of the Danube is 3,600 feet, 

 and its length 1,750 miles. From Passau to Vienna the fall is 1 in 2,200, from 

 Vienna to Old Moldova 1 in 10,000. Yet the velocity of the current from Vienna 

 to Basias (15 miles above Old Moldova) is ' from 2 to 3 knots an hour,' depending 

 on the amount of water. This would suffice to transport pebbles of the average 

 size of the English Bunter. Below the Iron-gates the fall is still less rapid, but 

 sand is carried down for a very considerable distance. If then the rivers of the old 



' I except floating pumice, cosmic dust, &c., as comparatively unimportant. 

 '^ See, on the subject of this paragraph, Daubree, Geol. Experiment., vol. i. sec. 2, 

 Ch. I., and J. A. Phillips, Q. J. Q. S. vol. xxxvii. p. 21, &c. 



