80 



Proceedings of the Royal Irish Academy. 



the Sliannon water, and making a deduction for the trifling organic 

 substances present, 20 grains of mineral matter i^er gallon^ out of 21 

 grains of solid stated in the results of analyses, would seem to be a fair 

 estimate. The analysis was recorded in June, and might be slightly 

 different from the average obtainable for the year. It is higher 

 than for the Thames water at Kingston, as might be expected, for the 

 Shannon area is for the most part limestone, and organic acids derived 

 from the peat of the vast tracts of bog would operate powerfully on 

 this rock. An estimate of 20 grains per gallon of mineral matter does 

 not err, however, on the major side, for an average was obtained by 

 Eischof for a number of rivers containing very small and very large 

 quantities of mineral matters, and is given by Sir A. Giekie as 21 parts 

 in 100,000, whereof 11 '34 parts were carbonate of lime.^ 



Twenty grains per gallon represents about 1,000,000 tons per 

 annum, conveyed to the sea from the entire Shannon area, equivalent 

 to 2740 tons for an average daily discharge of solid matter. 



Taking the Shannon yearly discharge to be approximately 9 inches 

 off each square inch of the drainage area, and 20 grains of mineral 

 matter to be carried away in each gallon, then in every 100 years an 

 average of 3*246 gallons flows off each square inch of surface, carrying 

 64' 92 grains of mineral matter. If we assume the rock affected and 

 eroded to be in great part somewhat earthy limestone, of 2*65 specific 

 gravity, the matter borne away would approximately total -ru-th inch, 

 about -3 foot in a century.^ Lowering of the surface at this rate 

 would be one foot in 12,000 years. The lowering of England and 

 "Wales was estimated to have been one foot in 12,978 years. 



It will be shown that a differential lowering of some 2500 feet 

 has taken place over most of the area now occupied by limestone in 

 Shannon basin since the commencement of the river's history ; which, 

 upon the basis of one foot in 12,500 years, or nearly 13,000 years 

 according to Mellard Reade's estimate, would have necessitated a total 

 period of erosion of more than 30,000,000 years. Such a figure seems 

 extravagantly high, particularly if, as we believe, the initiation of the 

 river-basin dates no further back in geological time than the Miocene 

 epoch. We must infer that the conditions of subaerial denudation 

 have been different in the past, including perhaps a much greater 



1 Bischof, Chem. Geol. i. chap, v., quoted by Sir A. Geikie, op. eit., p. 488. 



2 Mr. Mellard Reade's estimate for the general surface of England and Wales, 

 where not half the area is Hmestone, was "0077 foot per century. — Transactions 

 of the Liverpool Geol. Soc, 1882. 



