394 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [ June 1 9, 



111 taking the average compositions of ten examples of felspar, I 

 find that 66*2 per cent, consists of silica, 21 per cent, of alumina, 

 and 12-8 of alkalies &c. ; or, leaving out the alkalies, in 100 parts of 

 the sum of the silica and alumina, 75-86 would be silica, and 

 24-14 alumina — in round numbers, 3 of silica to 1 of alumina. But 

 in the plastic clays, taking 100 to represent the sum of the silica 

 and alumina, 65-59 would be silica, and 34-41 alumina, or less than 

 two parts of silica to one of alumina, a proportion that agrees very 

 closely with that of the silica and alumina in the average com- 

 position of a number of examples of chalk. 



14. On the Strtjctuee of the Postglacial Deposits of the South- 

 east o/ England*. By Searles Y. Wood, Jun., Esq., E.G.S. 



Some years ago I entered upon an examination of the Glacial and 

 Postglacial deposits of the East of England, and began to map the 

 general distribution of the beds upon the small Index Map of the 

 Ordnance Survey (10 miles to the inch), and, simultaneously with 

 that, during a regular geological survey, on the ordinary inch to the 

 mile Ordnance sheets 1 and 2. 



I was induced to select the latter sheets for this purpose, princi- 

 pally because they comprised that part of England where the most 

 important members of the Glacial and Postglacial series approached 

 nearest to each other, and therefore the part likely to throw the 

 greatest light on their 'relative structure — and in some measure 

 because my place of residence afforded facilities for the task. 



This survey, when I had made some progress, led me to believe 

 that the entire valley-system of the East of England originated in 

 centres of arc-like or curvilinear disturbance, which immediately 

 preceded the elevation of the bed of the sea from which was deposited 

 the wide-spread deposit of Boulder-clay forming the latest of the 

 Glacial beds of the South of England. My view is that, these dis- 

 turbances having given an impress to the surface, the denudation 

 accompanying and ensuing upon the elevation of the sea-bed, by 

 wearing more deeply the impress thus imparted, made it more 

 conspicuous in the parts where this denudation was most prolonged. 

 I found also that the rectilinear upheaval along the Wealden line of 

 disturbance appeared to have been a subsequent event to this curvi- 

 linear denudation, and that generally these rectilinear disturbances 

 had occurred long after those which accompanied the first elevation of 

 a part of the Glacial sea ; so that, while a part of the valley-sys- 

 tem was due solely to the action of the first of these agencies, the 

 rest (and more especially the part of England lying south and south- 

 west of London) had originated from the combined action of the 

 two. I very briefly, and in some respects imperfectly, called atten- 



* The portion of England to which these observations apply is that lying 

 east of a line drawn from the south-western extremity of the Wash to the valley 

 of the Severn, near Bristol, and thence south to the English Channel. 



