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city of the current, the motive power is increased 64 times (2^); if we treble the 

 velocity, the motive power is increased 729 times (31=) and so on. From this it 

 can easily be seen how immense a change in the character of the deposit of any 

 rock can be made by a comparatively small amount of increase, or decrease, in 

 the velocity of the current or river which is forming the deposit. Again it is 

 found that water moving at the rate of three inches in a second, tears up fine 

 clay, six inches in a second tears up fine sand, 12 inches in a second tears up 

 small pebbles, 36 inches in a second large gravel. Bearing these facts in mind, 

 let us, for a moment, consider a few of the known causes which may arise to 

 alter the velocity of a great river. And first and foremost would come, any, 

 even the smallest, alteration in the level of its bed ; if by the action of volcanic 

 force, the hills or table land through which any river flowed were either elevated 

 or depressed, the result would be either the great increase or great decrease in 

 the velocity of the river, and consequently the entire change in the relative posi- 

 tion of its deposits. Again, when two rivers meet in their courses, the result is 

 not so much the widening of the bed, as the increase of the velocity of their 

 current ; here again we must note a frequent cause for a complete change in the 

 nature of a river's deposit. Again, another important cause of the velocity of 

 any river is the rainfall in the country where it has its origin — that is, in years 

 of great rainfall the river will be much more rapid than in years of small rainfall, 

 and in the former case would bring down and deposit a much greater quantity 

 of material than in the latter case, and the deposit in after ages when viewed 

 by geologists might erroneously be supposed to be the work of either a much too 

 great or a much too short a time. And now, recalling to mind the laws of 

 moving waters, and the causes which I have briefly touched upon which may 

 operate to cause an alteration of velocity or volume in any stream or river, let 

 us apply them to the example which I proposed — namely, the Nile— and see how 

 our supposed conclusion as to the length of time required to form the Nile mud 

 deposit may be effected. And first, taking the case of the deposit of sand and 

 mud formed by the overflowing of the waters of the Nile, it can easily be under- 

 stood how, if under one of our supposed causes, when the rainfall over the up- 

 lands where the river has its origin was much less than at present, the overflow 

 of the river would be much less, and its deposit of mud would be much less, and 

 also it is quite possible— indeed probable— that the action of the river as regards 

 deposition might be intermittent. Here, then, we can at once recognise a pos- 

 sible source of error in any conclusions which we might have come to as to the 

 time of the formation of the Nile mud. Again, let us suppose that from one or 

 another of the causes which I have mentioned the river in former times had a 

 greater velocity than at present, and that its action had been brought to bear 

 upon the area of deposit we are supposed to be investigating, we should then 

 find that by the greater moving powers of the water, the fine mud which 

 might have been already deposited would have been torn up and borne away 

 out to sea, and if the action of the river was either considerably increased for 

 a short time or slightly increased for a long period, hundreds or thousands 

 of years of former deposits might be swept away. In this case, therefore. 



