498 A. Tylor — Formation of Deltas. 



feet), moved over the Continent of North America — is it so improb- 

 able that in this epoch of universal cold, the valley of the Amazon 

 also had its glacier, poured down into it from the accumulations of 

 snow in the Cordillera, and swollen laterally by the tributary 

 glaciers descending from the table lands of Brazil?" 



After this, Oscar Fraas's account of the remains of true glacier 

 moraines on Mount Sinai, at a height of 700 feet only above the 

 sea-level, does not seem so astounding as it really is. 



Prof. Forbes writes of the glacial sea (p. 382, vol. i., Mem. Geol. 

 Survey, 1843) : — "An arctic sea inhabited by a limited and uniform 

 fauna " (I presume Prof Forbes intended to restrict the uniformity 

 of the fauna to the zones of equal depth and temperature), "extended 

 from the then western coasts of Siberia into the heart of North 

 America, and southwards in Europe to the parallel of the Severn, 

 and in America to that of the Ohio." 



Prof. W. W. Smyth quotes an estimate of the temperature of the 

 summer months, of the s,pace now occupied by London, in the 

 Glacial Period at 126° F. The author calculates that a decrease of 

 30° Fahr. in the mean temperature of the ocean would reduce the 

 level of the sea 150 ft., while a deposit of snow or ice 1500 feet 

 thick in an area of land one-tenth of that of the sea would also re- 

 duce the level of the sea 150 feet. 



APPENDIX. 



Note on fhe theory of a "Pluvial Period," p. 486. — [Mr. Prestwich in all his writings 

 has attributed the valley gravels to floods caused by sudden melting of ice and snow- 

 under the maximum rainfall now met with in similar localities. There is no trace in his 

 works of the idea of a Pluvial Period until 1872. The average rainfall in some parts 

 of Great Britain being five times that falling in other parts, Mr. Prestwich has taken 

 the higher figure for his British Gravel Period, just as Sir C. Lyell refers to the 

 greatest eruption in modern times as a guage for all previous ones that have occurred. 

 Both Mr. Prestwich and Sir C. Lyell consider present meteorological conditions as 

 the test for those that previously existed. The writer, on the contrary, infers the 

 amount of rain causing the Quaternary deposits from the coarseness and position of the 

 gravel deposits themselves. Both plans may be useful in endeavouring to ascertain 

 the truth about a very difiicult question. Mr. Prestwich has, in his address of 1872, 

 quoted M. Belgrand, 1870, for a 20 or 25-told rainfall in the Quaternary Period, and 

 oiven his qualified assent to the idea of a Wet or Pluvial Period ; but he has not referred 

 either to A. Tyler's paper in 1853, in which he first proposed the hypothesis of many 

 times the present average rainfall and deposit, or to page 63, vol. xxv. Quart. Journ., 

 where A. Tylor calculated the volume of water in the Gravel Period as 125 times that 

 at the present time. At page 9, same vol., A. Tylor writes of 300 inches of rain at 

 the sea-level in the Quaternary Period. In the text of the paper, read November, 

 1868, and now published in the Geological Magazine, this subject is treated at 

 greater length. The term Pluvial Period was first used, page 105, vol. xxiv. Quart. 

 Journ., 1868, in a paper by the writer, and the term has been since used by Prof. T. 

 R. Jones, Prof. Dawson, and other writers. Mr. Prestwich never contemplated any 

 general excess of rainfall above what is known at the present time, and therefore 

 difi'ered from the author, who made an hypothesis of a general rainfall on the globe 

 many times more than at present in 1852. Mr. Prestwich opposed every view brought 

 forward by the author in 1852, and also in 1868 and 1869.]— Aug. 1872, A. T. 



Note on " Elevations," pp. 397 and 399.— [The marine shells at Moel Tryfaen and at 

 Coalbrook Dale at such different levels indicate a very unequal elevation of part of 

 England and Wales. The hypothesis of a change of sea-level accounts for a very even 

 change of position of land and water over large areas, totally differing from anything 

 that could be produced by elevations or subsidences, which are always accompanied by 

 flexures and uneven movement of strata.] — A. T. Aug. 1872. 



