Dr. Charles Ricketts — On Subsidence and Accumulation. 121 



by rivers. The great historian imagined that the area in which is 

 situated the Delta of the Nile was formerly a deep sea, which had 

 been gradually filled up by deposition from the river, and many 

 geologists consider that it is under such conditions that delta-accu- 

 mulations occur. Borings have been made in the case of the Po to 

 400 feet, in the Ganges to 481 feet, and in the Mississippi to 630 feet, 

 through strata generally fluviatile, having near the lowest depths 

 which were reached beds of turf and other vegetable matter, and 

 which, therefore, must at one time have formed a land surface. As 

 an alluvial plain is situated in an extension of a valley, and a delta 

 is an extension of the plain, being situated in what is a continuation 

 of the same valley, it follows that the area in which the delta- 

 accumulation occurs has, prior to the deposition, been excavated by 

 the same causes which have excavated the valley, and these have been 

 shown to be sub-aerial. As deltas and other evidences of subsidence 

 occur so generally near the mouths of great rivers, whenever any 

 great amount of sediment is brought down by the streams, it appears 

 impossible not to consider that the depression is dependent on and 

 caused by the accumulation ; that as layer after layer of mud and 

 sand has been deposited in the deltas and neighbouring seas, the con- 

 stant addition of fresh material causes by its weight a subsidence 

 which is generally gradual and imperceptible, but, under certain con- 

 ditions, may occasionally occur suddenly and be accompanied by 

 earthquakes, as in the basin of the Mississippi near New Madrid, in 

 1812. 



The production of Bays has not unfrequently been attributed to 

 the effects of waves and marine currents ; but such a theory when 

 applied, as it has been to the Gulf of Mexico, pre-supposes so great 

 an erosive action as to have excavated the land to depths of from one 

 to two miles, as well as the redistribution to some other locality of 

 the materials removed. It will be seen that in almost every instance 

 bays have rivers flowing into them, and appear as if they form a 

 continuation of their valleys, being wide in accordance with the 

 width of the valley ; also, contrary to what might be expected, the 

 water at least sometimes, as in the Ganges and Mississippi, becomes 

 very rapidly deeper where the deltas are greatest, and the greatest 

 amount of sediment is being deposited. To account for their forma- 

 tion it will require the occurrence of a subsidence of the land to a 

 greater extent than the valley can be filled up by the sediment 

 brought down by the river. 



An Estuary may be described as a bay situated in what was once 

 a narrow valley or gorge, and like a bay is the result of subsidence 

 of the land, by which the river-bed has been depressed below the 

 sea-level so that the waters fill it up, in a similar manner to what 

 occurs if an embankment is constructed across a similar narrow 

 portion of a valley, with the addition of the changes which occur in 

 the level of the water according to the state of the tide. 



It is evident that the agency by which the estuary of the Mersey 

 has been excavated has been sub-aerial. The smoothed, planed and 

 striated surfaces which are found on each side of the valley beneath 



