and on the Basins of London and Hants. 127 



at the highest levels on the summits of the hills^ whilst others have been drifted 

 to the adjacent valleys, and sometimes, as at Portisham and Sidmouth, beyond 

 the present area of the chalk*. 



The general dispersion of these enormous blocks, which probably once 

 existed as concretions in extensive beds of sand, since swept away in conse- 

 quence of their buoyancy, affords strong evidence, in addition to that supplied 

 by the residuary patches and outliers of plastic clay, that these tertiary forma- 

 tions were once deposited over nearly the whole surface of the chalk in the 

 south of England ; and that their separation into the two distinct basins of 

 London and Hampshire has resulted, partly from local elevations and depres- 

 sions by subterraneous violence, since the deposition of the plastic clay; and 

 partly from the still more recent removal of much of their substance by dilu- 

 vial denudation. 



I cannot leave this subject without referring to the remarkable fact of the 

 occurrence of insulated portions of tertiary strata, as well as of chalk and green- 

 sand, on the summits of the Savoy Alps, at elevations of more than 10,000 feet 

 above the level of the sea ; which seem to bear the same relation to the ter- 

 tiary strata of the valleys of Italy, France, and Germany, that our trifling 

 Heights of Inkpen in Hants, Blackdown in Dorset, and the North Downs in 

 Surrey, bear to the lower regions of the Basins of London and Hants. As these 

 Alpine deposits are contemporaneous fragments of the more extensive strata of 

 the adjacent low countries, we are forced, in explaining their present position, 

 to adopt one or other of two conclusions ; either, that at the time of the deposi- 

 tion of these strata the sea covered, not only the highest portions of the chalk 

 of England, but also the summits of the Savoy Alps ; or, that since the deposi- 

 tion of these beds, by elevation of the mountains or depression of the valleys, or 

 by the united effect of both these causes, the relative level of the one to the other 

 has been changed to an amount of many thousand feet. Now as the undisturbed 

 and nearly horizontal position in which the tender and frangible materials of 

 the tertiary strata still remain in the basins of Paris and Lombardy forbids us 

 to suppose that any depression could have brought them down so quietly to 

 their present level, the theory of the elevation of those few portions which 

 occupy the Alpine summits, remains by far the most probable that is submitted 

 to our choice. 



The idea first suggested from the examination of the basins of Paris and 



* These blocks at Sidmouth afford a beautiful example of a siliceous breccia of the same age 

 and character with the well known Hertfordshire pudding-stone ; and agreeing with some varieties 

 of it in the circumstance of the chalk flints remaining angular, and not being rolled to pebbles, as 

 more frequently is the case. 



