RIVERS OF NEW SOUTH WALES. 391 



feet ; but in none of them was any indication found that the 

 bottom of the alluvium was near. Under the title of " A River 

 of Ruined Capitals," the manner in which the Hooghly alters 

 its course while the delta at its mouth is being extended seaward, 

 has been graphically described by one of the most brilliant of 

 recent Anglo-Indian writers. The title is a suggestive one and 

 recalls to mind that ruins of ancient cities are among the most 

 substantial evidences of the important changes which are due to 

 the alteration of the sources of several of the rivers of Upper 

 India. 



Some conception of the time required to accumulate such deposits 

 as those of the Indo-Grangetic Valley may be obtained by examina- 

 tion of the rates at which deltas advance seaward. It is known 

 that in the period of three hundred years ending 1869, the delta 

 of the Tiber advanced a distance of five thousand five hundred 

 and twenty feet, or at an average rate of about eighteen and a- 

 half feet per annum. The deposits at the Sulina mouth of the 

 Danube and at the South Pass of the Mississippi, ai'e advancing 

 seawai'd at a rate of from sixty to one hundred feet annually, and 

 it is noteworthy that at the unimproved mouths of these rivers, 

 the rate of advance is more than three times as great. 



While these considerations give some clue to the length of time 

 which may have been taken in the formation of deltas, it is to be 

 remembered that the rate at which a delta advances is necessarily 

 iri'egular, depending as it does on the varying conditions of a 

 river. An important element in the formation of deltas and one 

 which is generally overlooked, is the destruction of microscopic 

 life at the place where the fresh and salt waters meet. A well- 

 known authority, Mr. Baldwin Latham, M. Inst. C.E., states that 

 in the case of rivers flowing into tideless seas, as in the instance 

 of the Nile, the formation of the delta is due even more to this 

 cause than to the solid matter brought down. 



In applying to the rivers of New South Wales, the principles 

 already discussed, we find that in relation to their silt-carrying 

 capabilities, these rivers may conveniently be divided into two 

 classes. The first comprises those whose transporting power is 

 suflicient in an average series of years to maintain the sectional 

 areas of their channels undiminished and to carry a great propor- 

 tion of their silt to the ocean, or to the river or lake, into which 

 they flow. The second class includes those rivers whose section 

 beyond a certain point diminishes as the distance from the source 

 increases, and whose silt is deposited chiefly along the lines of 

 their courses. All the coastal rivers may fairly be ranked in the 

 first class, while all the western rivers with the exception of the 

 Murray, must be ranked in the second. 



It is clear that the natural tendency of a river of the first class 

 is to alter gradually to one of the second ; for as denudation pro- 

 gresses and the catchment area wears down, the rate of fall of 



