A. Tylor — Formation of Delias. 487 



the tract situated within the water-shed, consisting of unequally in- 

 clined, unequally hard, and of unequally pervious strata, start and 

 determine the subsequent direction of every watercourse, and all 

 main, tributary, and pluvial denudation ^ in the valleys of the Po, 

 Weald, Thames, and all other rivers. Thus tributaries and side 

 streams, flowing off the soft Oolites near Schaffhausen, from their 

 inclination against the stream flowing from very high ground, oppose 

 and repel the Khine, while others further on, inclining towards 

 the stream, attract and assist it to pierce the much harder rocks "be- 

 tween Bingen and Bonn.^ Thus the hard rocks sometimes are 

 denuded, while soft ones out of the track escape. 



M. Lombardini, in his work "Changes in the Hydraulic Conditions 

 of the Po"* (1852, Milan), removes some misconception about the posi- 

 tion of the bed of the river Po, which have not only passed current for 

 many years, but have been quoted as typical conditions of rivers. 

 M. Lombardini shows that the flood high-water mark of 1839 was 

 only 5 feet above the ancient natural bank, and was 3 feet actually 

 below the surface of the ancient embankment of the Po. The pro- 

 longation of the Po, as erroneously calculated by M. de Prony, was 

 from A.D. 1200 to a.d. 1600, at the rate of 815 feet per annum, 

 from A.D. 1600 to the present century, at the rate of 227 feet per 

 annum. M. Lombardini shows that the rate of progress of the 

 Delta from a.d. 1600 to the present day is only one-fifth greater than 

 formerly, and that increase is owing to the levees, which have raised 

 the level of the river 3 feet 3 inches. 



There appears to be no foundation whatever, according to Lombar- 

 dini, for the statements made about the condition of the river Po, 

 for Lombardini proves by reference to flood-gates that the extreme 

 low-water surface of the river lias not changed sensibly in two 

 centuries ; also that at Stellata, 16 miles above Ferrara, the surface 

 of the river at that point could not have been elevated since that day 

 by the prolongation of the Po. There are no lumps of clay in the 

 Po, raised by subsidence elsewhere, as in the Mississippi. The drain- 

 age of a lake, or melting of snow, or denudation, or deposition, 

 disturb the existing equilibrium of the crust of the earth, but gravity 

 must have always assisted the force producing subsidence, and 

 opposed that producing elevation. If the mass of freshwater strata 

 from the earliest period has always borne the same proportion to 

 marine strata above the sea-level that it does now, and that the mass 

 of earth above the sea-level has always borne to the mass of the sea, 

 then extra heat must have been obtained from above and below, or 

 from chemical action,Ho produce elevate ry and depressing force exactly 

 sufficient to do the required work, less the effective force of gravity 



1 Denudation is -work done to produce stability in strata to bear their load under 

 the new physical conditions established. 



- The author has a very interesting case of a river piercing a rock near Istrad Yellte, 

 South Wales, which he wiU shortly describe, showing an actual case of denudation. 



2 See, "Kemarks on Denudation," by the Author, read May, 1868, p. 71, vol. 

 xsv.. Quart. Journ. All tributaries adjust their bottom levels in relation to the 

 main stream. They cannot cut down their beds when the main stream blocks their 

 water. * Humphreys and Abbot, page 414. « See Sir H. Davy. 



