A. Tylor — Formation of Deltas. 495 



By knowing the direct distance from any point, you can determine 

 the height of any other spot correctly, that is within six feet, or, by 

 knowing the height, you can find the distance within a few miles 

 between any points. 



The Volga. — For a great part of its course of 2,000 miles long, 

 this parabolic curve truly represents the contour of the surface 

 of the alluvial plain and Delta of the Volga. The Volga rises in 

 a lake 630 feet above the level of the Caspian Sea. 



Mr. Eyth, who has superintended irrigation works in Egypt, and 

 is well acquainted practically with the Delta of the Nile, considers 

 that the surface of the Delta is a mathematically exact curve, and 

 thinks its straightness at the Delta portion, where he has particularly 

 observed it, makes it more like an hyperbola than the parabola I 

 suggested. 



The engineers in the Mississippi Eeport treat that river as a plain, 

 but their own table of the changes in level per mile in passing from 

 Balize to St. Louis proves that it is a curve. See ante page 489. 



The flatness of the Nile cannot be greater than that of the Missis- 

 sippi, which, according to Mr. Meade (Appendix A. xix. Mississippi 

 Eeport), is If feet in the last twelve miles of its course, or If inches 

 to the mile on the average. 



We have clear indications that these stratified gravels were de- 

 posited under very different meteorological and physical conditions 

 from any we now meet with. We have found evidence of littoral 

 conditions in Delta deposits and on the sea-bottom down to the 100 

 fathom line, wherever examinations have been made ; these are only 

 in scattered localities, and the evidence is only obtained by boring 

 and dredging. 



Such littoral deposits as we have been considering are small indeed 

 compared with the littoral deposits to be seen in the Pacific Ocean 

 formed by coral zoophytes over a tract of 5,000 miles in length.' It 

 is stated that many species of these zoophytes would be destroyed 

 by immersion under water only a few fathoms deep. In ordinary 

 cases reef-building polyps do not flourish at greater depths than 

 20 to 30 fathoms. (Page 86, Darwin's Greological Observations.) 

 Exposure to air for a few hours at the surface of the sea is also fatal 

 to their existence. Mr. Darwin writes, " With respect to subsidence, 

 I have shown in the last chapter that we cannot expect to obtain in 

 countries inhabited by semi-civilized races demonstrative proofs of a 

 movement which invariably tends to conceal its own evidence." 

 (Page 127, Darwin's Greological Observations, 1851.) It was from 

 this difficulty of absolutely proving subsidences of sea-bottoms that 

 Mr. Darwin considered he had exhausted every demonstrable course 

 before he adopted the theory of subsidence to explain the formation 

 of the coral islands of the Pacific. At page 14:7 he speaks of sub- 

 sidences as "the one alternative." 



In the South Sea Islands every stranded root of a tree was 

 examined for the hard stones attached to it.^ It is therefore 



1 According to the accounts of early travellers. 



