A. Tylor — Formation of Deltas. 499 



Note on pp. 393 and 492. — [The author has sent full particulars of the slope of the 

 Mississippi, Ganges, Ehine, and Indus, to the Institution of Civil Engineers, Feb., 1 872.] 



Note on page 397. — [The author cannot find any evidence whatever as to the real 

 character of the rocks below the surface of the Pacific in the works of any geologist. 

 It is quite an assumption, unsupported by any evidence, to state that they are formed 

 of recent coral to considerable depths. It seems probable that reef-building coral 

 zoophytes must have lived at much greater depths than has been supposed, or they 

 could not have passed from one island to another, as they must have done to spread 

 over such a mass of islands separated by seas of enormous depth. The low tempera- 

 ture of our atmosphere and of the Atlantic at great depths is probably a survival or a 

 result of the Glacial period. In a warm period coral zoophytes might live at very 

 great depths, as they are said to do in the Eed Sea now. The supposition of 

 immense continuous periods of subsidence, without a counterbalancing elevation, is 

 against the principle of the conservation of force. There is no direct evidence of 

 EVEN elevation or even subsidence over any area of more than a few miles recorded 

 in the writings of geologists. The strata are always curved where there is direct 

 evidence of elevation or subsidence.]— A.T. Aug. 1872. 



Note on page 495. — [It requires further examination. In the Nile valley there 

 are now no constantly flowing tributaries, but there are immense side valleys, which 

 must have been fall of water in the Pluvial Period.] 



Note on the Glacial Period. — [The geological teacher who takes his pupils on the 

 field has a difficult task. Every valley almost that he can find is a case of pluvial 

 and fluvial action, the evidence of ice action being rare in many districts. Yet the 

 teacher has to teach the existence of the Glacial period by what is really the evi- 

 dence of a Pluvial period.] 



[The author has published this paper as read, with some slight alterations. He 

 thinks it alludes to some points which should be more carefully considered by geologists 

 than they have hitherto been. Fig. 2, Plate XI., has been corrected as to heights from 

 the drawing exhibited in 1868. Fig. 3, Plate XI., is altogether new, and is inserted as 

 necessary to explain Fig. 2 and the position of the Dour river referred to, in 1868, 

 p. 396. Fig. 4, Plate XL, refers to the Binomial Curves spoken of in 1868, but not 

 drawn, see note, p. 486.] — A.T., Aug,. 1872. 



*** The following note should have appeared on page 395, after Kne 12 from bottom : — The 

 shape and height of any hill is dependent upon its power of resistance. Where the base is clay, 

 exposed to the weather, or water action, either of the sea or brooks, the power of resistance is 

 comparatively small. The presence of a thin band of clay in a position in which it could slip, 

 might and has undermined and caused the denudation of strata thousands of feet thick in the 

 Wealden area. 



EXPLANATION OF PLATE XL 

 Note by MM. Degousee and Laurent, in Explanation of Fig. 1, Plate XL 



The soil of Venice is only one metre above the high-water mark of the Adriatic. 

 Most of the ground has been recovered from the deposit of alluvium which we have 

 explored at twenty diiferent points. 



The first and the last beds of this deposit, of which the thickness is not less than 

 172 metres by the well-boring of the Ca di E)io, present a great similarity throughout. 

 Nevertheless, the first beds contain shells generally all of marine species, together 

 with fragments of human industry. 



The lower beds contain fluviatile shells, intermixed more andmore with marine shells. 



The sands predominate towards the inferior part, and are frequently mixed with 

 fragments of ligmte disseminated through their mass. In the middle and upper parts 

 these lignites occur in thin bands, and in beds alternating with clays or sands. Mica, 

 very abundant in the lower part, is less so at the commencement of the deposit. 



One may say that the beds of this deposit are almost horizontal throughout the area 

 which we have explored. Thus, the two gre;it lines which intervene between the 

 soundings are — 1st, of Guidecca to S. Francesco della Vigna, about two kilometres ; 

 2nd, of Malghera to S. Servolo, about ten kilometres ; it was to this last that we 

 referred in our remark that it did not present any very great slope. 



One would almost think the Venetians dreaded the scarcity of water, in the event 

 of blockade, when one notices that everywhere there is abundance of spriiio- water 

 near the surface. 



At Sta. Maria Formosa one finds five water-bearing beds, of which the first is at a 

 depth of 250 metres. At St. Leonardo the water is found jetting up, at a depth of 

 twenty-four metres. If at other points these dilferunt principal sources are not 

 indicated, it is because they were of no importance, or were passed by unobserved ; but 

 it is probable that almost all these existed higher or lower. 



