260 PEOCEEDINGS OP THE CfEOI-OaiGAL SOCIETY. [Apr. 16, 



of the Geological Survey of Great Britain. As that sheet, which 

 was puhlished in 1857, is not illustrated by a memoir, like those 

 descriptive of many of the sheets of later date, and as some changes 

 were made in the mapping of the Tertiary beds in its north-eastern 

 part in 1859, I shall give a short notice of the ten Tertiary outliers 

 that have been there mapped. The country included in the S.E. 

 corner of the sheet to the north (Sheet 34) will also be noticed. Here 

 also the Tertiary beds have been resurveyed, which has made needful 

 some corrections in the next edition of the Memoir illustrating that 

 sheet. 



The Tertiary beds that are found in this district are — the ^* Lower 

 Bagshot Sand," the " London Clay," and the " Woolwich and 

 Reading Beds " (or, for shortness, the *' Beading Beds "). It wiU be 

 convenient to work from the east westward, and to notice all the for- 

 mations together, as they occur, instead of treating of each separately. 



It is well first to state that it would seem that, when Mr. Prest- 

 wich examined this district, before the publication of his papers on 

 the Lower Tertiary beds, sections were neither so plentiful nor so 

 clear as when the Geological Survey was in progress (1858-59). 

 Thus Mr. Prestwich says (in 1850), " The first" (that is to say, the 

 most westerly) ^' point where we meet with some uncertain indications, 

 without sections, of the basement-bed of the London Clay is capping 

 the summit of Bagshot Hill, between Great Bedwin and Hungerford* '' 

 (Map 12); and again (in 1853), "In Marlborough Forest the Ter- 

 tiary beds are so thin, and so disturbed by, or mixed with, drift, that 

 no good section can be obtainedf." I shall show that there is 

 London Clay three miles or more to the west of Bagshot Hill, and 

 moreover that the Bagshot Sand ranges still further westward. 



Tertiary outliers in Sheet 14 of the Geological Survey Map. — At 

 the western edge of the map (14), east of Great Bedwin, there are 

 three patches of the Reading Beds, the middle one capped with 

 London Clay, forming parts of a large and well-marked outlier, the 

 greater part of which is in the map to the east (Sheet 12). There 

 is a section of the Reading Beds in the brickyard at Folly Farm, and 

 northwards there are two other brickyards, the pits in which show 

 the junction of the London Clay and the Reading Beds. An account 

 of these sections will be found at p. 2Q of the Geological Survey 

 Memoir illustrating Sheet 12. 



At Castle Hill, south of Great Bedwin, there is an outlier of the 

 Reading Beds, probably capped by London Clay at the top of the 

 hill (judging by its height alone, there being no section of the latter 

 formation). This outlier, which is about a mile and a quarter in 

 length from north to south, but nowhere half a mile in breadth, is 

 well marked ; the Tertiary beds, for the most part covered with wood, 

 rising sharply from the Chalk. On its eastern side, in a chalk- 

 pit half a mile a little E. of S. of Broil Farm, there may be seen an 

 irregular junction of the " bottom-bed $ " of the Reading Beds with 



* Quart. Journ. Gbol. Soc. vol. vi. p. 257. t Ibid. vol. x. p. 85. 



X For an account of this bed, see Memoir illustrating Sheet 13 of the Map of 

 the Geological Survey of Great Britain, p. 23. 



