Revleivs — Dr. Ricketts — OnValleys, Deltas, etc. 89 



in shallow water and near the coast, subsequent accumulations of 

 great thickness prove a subsidence of the land, and the consequent 

 deposition of the succeeding strata further and further away from 

 the shore, even " at a distance of many miles from the coast, so 

 that by no possibility could the sediment have been derived from 

 marine denudation acting on cliffs." With this last argument we 

 cannot entirely agree, as no allowance seems to be made for the 

 dispersion of sediment by marine currents. 



Dr. Eicketts reviews some of the principal opinions in regard to 

 the origin of valleys, and advocates their subaerial origin. He refers 

 to the eroding power of ice, and gives his opinion that the subsidence 

 of the land in England during the Glacial period was due "to the 

 combined weight of ice and the Boulder-clay pressing down the 

 surface to below the sea-level ; the land being again raised to a con- 

 siderable extent, when, upon the return of a more genial climate, it 

 was relieved of its load of ice and snow." 



Turning his attention to Deltas, he remarks that these accumula- 

 tions, which attain siich a great thickness in many instances, must 

 have been accompanied by a subsidence of the area over which they 

 are deposited, " which is generally gradual and imperceptible, but 

 under certain conditions may occasionally occur suddenly." This 

 depression is, he thinks, dependent upon and caused by the accu- 

 mulation of sedimentary matter. 



These notions put forward by Dr. Eicketts, " that alterations of 

 the level of the surface of the land are in very many cases due to 

 alterations in the amount of pressure, either from accumulations of 

 material on, or from denudation of, the earth's surface," are very 

 suggestive, but we cannot regard them as adequately explaining the 

 facts to which he has alluded. Dr. Dawson, as he remarks, thinks 

 that during the Laurentian period, the accumulation of sediment on 

 the " still thin crust of the earth " weighed down the surface, and 

 caused great masses of the sediment to come within the influence of 

 the heated interior nucleus, and thus extensive metamorphism took 

 place ; but this does not help us to explain present movements. 



In regard to Bays, which are generally attributed to marine 

 denudation. Dr. Eicketts remarks that with few exceptions they have 

 rivers flowing into them, and appear as if they formed a continuation 

 of the valleys ; and when deltas occur, it will require the occurrence 

 of a subsidence of the land, to a greater extent than the valley cetn 

 be filled up by the sediment brought down by the river. He there- 

 fore thinks that in such cases the river systems were formerly 

 greatly extended, receiving as tributaries rivers which now empty 

 themselves into the bays. Estuaries, he also looks upon as the 

 result of a subsidence of the land, by which the river-bed has been 

 depressed below the sea-level, owing to the accumulation of sediment. 



In conclusion. Dr. Eicketts points out instances of depression which 

 he thinks may be traced to the pressure of accumulations which have 

 not been dependent on river-action. He refers to the harbours along 

 the coasts of Sussex and Hampshire. At Spithead large accumula- 

 tions of flinty shingle, gravel, and sand have been formed; and such 



