A. Tijlor — Formation of Deltas. 485 



n. — On the Formation of Deltas : and on the Evidence and 

 Cause of Great Changes in the Sea-Level during the 

 Glacial Period. 



By Alfred Tyloe, F.G.S. 



(PLATE XI.) 



Part II. — Continued from page 399, 



IT would Tbe tlie safer plan, in considering the remarkable Gravel 

 and Crag deposits which characterize so distinctly the Quaternary- 

 Period, to infer the size of rivers, amount of rainfall, and elevation 

 of tides from the deposits themselves. Further acquaintance with 

 meteorological phenomena may find a fitting explanation of the 

 difficulties we meet with in explaining the position of the gravel at 

 such heights above our present streams, and fresh- water, alternating 

 with marine clay, and sands at such depths below the sea-level. 



Delta of the Po. — ^The diagram, PI. XI., Fig. 1, represents the dif- 

 ferent strata passed through in making 20borings for water in the Delta 

 of the Eiver Po, near Venice. These extend from St. Servolo to Ghetto 

 Nuovo, a distance of about 6 miles. The well at Casa de Dio reaches a 

 depth of 572 feet. That of St. Maria Formosa 452 feet. M. Laurent, 

 of Paris, the experienced well-borer, who is well acquainted also with 

 geology, kindly made the drawing for me many years since, a copy 

 of which I now use as a diagram. Of course the only points accu- 

 rately determined are those in the 18 borings. The two great water- 

 bearing beds have been traced throughout the 18 borings, and are 

 represented as continuous in the diagram marked A. and B. They can 

 only supply water equal to the quantity of rain on their outcrop. 



The clays, lignitiferous sandy pebble and shell beds, are represented 

 by different colours ; also the wells from which gases escape are dis- 

 tinguished on the drawing Fig. 1. At St. Leonardo, No. 17, the lig- 

 nitiferous beds occupy nearly one-third of the first 190 feet of strata 

 passed through. The great water-bearing sands are 40 feet thick at 

 the A. point. 



The lignites sometimes succeed clays, sometimes sand, and are not 

 continuous, they are therefore entirely distinct in stratification from 

 the true coals of the Carboniferous formation, which are stratified con- 

 tinuously and usually repose upon clay. It is however interesting to 

 observe the horizontal development of the water-bearing sands of a 

 Delta, which may compare with similar horizontal bands in the Coal- 

 measures. The evidence offered by these two remarkable beds of 

 sand is that there was a marked change in the amount of denudation 

 in the area drained by the Po, or in the material thrown back on to 

 that part of the Delta by the sea, arising from a change in the posi- 

 tion of the margin of the Delta (when these sands were deposited), 

 as well as the coarse material indicating a greater rainfall. 



The surface of the soil at Venice is 5 feet, on an average, above 

 high- water mark in the Adriatic. 



The mineral character of the first and last stratum perforated at 

 Casa de Dio are remarkably alike, although separated by 500 feet of 

 strata. The ujoper bed contains fragments of human industry and 

 marine shells exclusively. In descending from the surface the 

 marine shells become mixed with freshwater species. Sands are 



