The Geology of the Marble Delta. 665 



within that of the Lower Greensand (neighbouring). The local 

 constituents are Reading or other Tertiary pebbles, and flint ; the 

 latter is small in quantity where the gravels rest on Tertiary beds, 

 but much increased where on the Chalk. In addition, pebbles of 

 sarsen are not uncommon ; they have been wrongly identified as 

 quartzite. The pebbles to which the term ' neighbouring ' is ap- 

 plied consist of white quartz and lydite, all small ; they have been 

 proved to have been derived from the Lower Greensand, and this 

 identification is corroborated by the fact that over considerable 

 areas small fragments of chert from the Lower Greensand are 

 associated with them. 



' Far-travelled ' stones, derived from the Bunter, Carboniferous 

 Limestone, Red Chalk, etc. are entirely absent from these deposits 

 as originally laid down. 



So long as they are composed mainly of small pebbles the gravels 

 keep at a nearly constant level over a very wide area, a little more 

 than 400 feet above sea-level. This type has once covered the 

 greater part of the Thames Valley that is now at or below this 

 level. As they rise to a greater height the Tertiary pebbles (and 

 flints if present) increase in size. 



Outliers of the finer deposits have recently been met with near 

 Chorley Wood, and on both sides of the Misbourne a little above 

 Chalfont St. Giles. The steady increase in the proportion of small 

 quartz-pebbles in this area suggests that they must have entered 

 the district by the Wendover gap, at the head of the Misbourne 

 valley ; this has been proved to be the case. 



The coarser gravels of this composition occur on the south side 

 of the Thames, at varying heights, up to above 600 feet ; these all 

 rest on the Chalk. The author has pointed out that there must 

 be corresponding coarser gravels also resting upon the Chalk on 

 the north side of the Thames, and the occurrence described by 

 Mr. Gilbert now shows that this is the case. 



The higher gravels on the south side, of the Thames seem to be 

 allied to the Lenham Beds, and this greater age is supported by 

 the considerable denudation that has taken place since this series 

 of gravels was deposited. The post-Glacial denudation of the area 

 about London, away from the immediate neighbourhood of the 

 Thames itself, is quite small in comparison with it. 



February 5th.— Mr. G. W. Lamplugh, F.R.S., President, 

 in the Chair. 



The following communication was read : — 



'The Geology of the Marble Delta (Natal). 5 By Alexander 

 Logie Du Toit, B.A., D.Sc, F.G.S. 



The paper deals with the crystalline dolomitic marbles of Port 



