486 A. Tijlor — Formation of Deltas. 



most frequent in the loAver beds, and contain lignite, consisting of 

 broken pieces of vegetable origin, distributed through the mass of 

 sand like some beds in the Hastings Wealden, where subsequent 

 denudation enables the relations of clay beds to sands to be well seen 

 (seeA.Tylor,ontheWealden,Q.J.Geol. Soc, I862,vol.xviii.,p.250). 



In the middle and upper series the lignite occurs in veins or threads, 

 and in beds alterna,ting with sands and clays. The lower beds are very 

 micaceous, but there are only traces of mica in the upper series. 



The beds in this Delta may be said to be horizontal, as in a distance 

 of six miles there is no sensible difference of level. In the Wealden 

 there are numerous flexures.^ 



At St. Maria Formosa, No. 8, there are 5 water-bearing beds, one 

 within 8 feet of the surface. At St. Leonardo the spring of water is 

 80 feet from the surface. There are great variations in the water- 

 level of springs and wells in most deltas. 



The course of the river Po from its source at Monte Viso, 801 feet 

 above the mean level of the sea, to the mouth at Maestro, has 

 been drawn by Mr. Beardmore (Man. Hydro., plate xiii.), who also 

 gives sections of the tributaries. 



For a distance of 150 miles between Chyvasso and Cremona the 

 line represented by the level of the bank of the Po closely coincides 

 with the parabolic curve ; at no part is there a greater error than 5 feet 

 in a course of 150 miles. 



1 mention these circumstances, as Deltas and alluvial formations are 

 described as planes, while their surface really is a close approximation, 

 I believe, to a true parabolic <5urve. It is sufficient here to record the 

 fact that the approximate level of the bank of the Po at any point can 

 be ascertained, if three points are given in the manner above stated. 



M. Ch. Laurent writes to me, Oct. 31, 1868 :— " The wells of 

 Venice, are, as you know, in the enormous Delta formed by the Po, 

 the Adige, the Tagliamento, and the Brenta, filling up a gulf. The 

 alternations of sand met with often present debris of marine shells, 

 Cardium, etc. The lignites and lignitiferous clays, as far as I could 

 observe, in the well sunk in the Island of San Servolo and at the 

 Madonna del Orto, contained fresh-water debris and terrestrial 

 remains. In this debris I have seen Succinea, Pupa, Helix, and even 

 in the lignites in the well sunk in the Place St. Marc, an elytra, 

 perfectly preserved, of an insect (Buprestis). This insect was un- 

 fortunately destroyed by contact with the air. There is in these 

 deposits an oscillation or alternation (balancement) between the 

 marine products and the fresh- water products. It is altogether what 

 takes place now at the surface in modern deposits." 



Flexures ^ arising from the original or . (latest) uneven upheaval of 



^ Not fractures exclusively (see -Quart. Journ., p. 251, 1862, by the author). The 

 author thinks these flexures are in consequence of uneven upheaval, often in binomial 

 curves, along and at right angles to a curved axis, elevated most in the Pluvial Period 

 at Crowboro', and before denudation commenced; flexures, longitudinal and transverse, 

 aided by side streams, principally guide the Wealden watereourses and rivers in their re- 

 markable course ; this I shall prove by measured sections hereafter, (See Figs. 6 and 6.) 

 —A.. T., Nov. 1868. 



2 Flexures in binomial curves may be seen in all Geological books, although they 

 are not described as such by those who have drawn them. 



