268 Mr. S. V. Wood on the Events which produced and terminated 

 gesimal division of the circle. The square root of 



16 16 J96 2_ _64_ _J_ J_ 1 



9" ~ 3000 + 10 7 10 7 + 10 10 + 16-10 9 7-10 11 ~*~ TOO 15 



true to the sixteenth decimal. 

 London, February 23, 1863. 



S. M. Drach 



P.S. ^2- s/ (•93 2 + -85 2 )= 0-00000037 only, a very near 

 approach to the geometrical representation of the side of the 

 double cube. 



XXXIX. On the Events which produced and terminated the Pur- 

 beck and Wealden Deposits of England and France, and on the 

 Geographical Conditions of the Basin in which they were accu- 

 mulated. By Searles V. Wood, Jun* 

 [With a Map, Plate V.] 



THE Wealden deposit of the South-east of England has, from 

 the time of its being brought into prominence by Dr. Man- 

 tell, been a fruitful subject of discussion, and has been indiffer- 

 ently referred to as a formation produced by a lake, a river, a 

 delta, or an estuary, as the view of the author inclined ; but the 

 precise geographical conditions under which the deposit was pro- 

 duced have not, so far as I am aware, yet been made the subject 

 of a separate essay. 



In a paper read before the Geological Society in the year 1846, 

 upon the so-called Wealden of Sutherlandshiref, Mr. A. Robertson 

 entered upon a consideration of the conditions under which what 

 he termed the freshwater deposits of the Portlando-Neocomian 

 epoch were accumulated; and although he did not recognize 

 that division between the Purbeck and Wealden which was 

 afterwards established by Prof. E. Forbes, and has been since 

 recognized, and refers to the same age formations that are now 

 regarded as distinct, yet his views of the manner in which the 

 basins containing these freshwater deposits were formed, if 

 applied to the case of the principal one, that of the South-east 

 of England, and if the peculiar conditions attaching to that 

 area are considered, appear to offer an explanation of the phe- 

 nomena under which the great freshwater deposit of Portlando- 

 Neocomian age in England was produced. Mr. Robertson did 

 not attempt to enter upon the consideration of these condi- 

 tions in detail, but in general terms argues that the forma- 

 tions under discussion were deposited in closed basins, once 



* Communicated by the Author. 



t Quart. Journ. Geol. Soc. vol. iii. p. 113. 



