INTRODUCTION xliil 



surface soil, and influences the vegetation. Moreover, these groups lie 

 one above another in an order which is always the same. But in other 

 cases there lie above the bed-rock accumulations of sand, gravel, or 

 clay, which may be distinguished as Superficial Deposits. These rest 

 sometimes on one and sometimes on others of the group of stratified 

 rocks, from the oldest up to the youngest, and are therefore later 

 than the newest of the stratified formations. Though nowhere 

 reaching any great thickness, they are often thick enough to be the 

 determining factors in fixing the character of the soil and of the plants 

 that grow on it. This fact has been mentioned in passing several 

 times, and in the case of the Chalk the Superficial Deposits have been 

 described at some length. 



It is time now to say a word about the other members of the group. 

 A large number of these may, from the point of view of the botanist, 

 be conveniently classed together, because they are mainly gravelly in 

 character. All are certainly younger than the Bagshot Beds, and it 

 is likely that all are Post-tertiary, but it is not possible to fix more 

 closely the date of many of them. In some cases the pebbles or 

 angular fragments which they contain are mainly of flint ; in other 

 eases they include rocks that have come from a distance, such as 

 Quartzite, White Quartz, and Igneous Rocks. Frequently they occur 

 as capj)ings to hill-tops, but they are not confined to such situations. 



In the neighbourhood of Oxford these deposits make no marked 

 feature in the contour of the country, but they are found on much of 

 the higher ground. Wytham Hill, the north side of Cumnor Hurst, 

 Bagley Wood, and other places, have a pebble drift, largely composed 

 of Quartzite similar to that which forms the conglomerate beds of the 

 New Red Sandstone. A coarser drift, containing large masses of 

 Quartzite, Gritstone, and Felstone, with fragments of Porphyry, Gneiss, 

 and Red Quartzite, also occurs. Gravels also belonging to this series 

 cover the ground between the Thames, Kennet, and Pang, as at Cold 

 Ash, Frilsham, and Yattendon, and there is a thick deposit of them 

 on the common of Bucklebury, forming the crest of the hilly ground 

 and capping the Lower Bagshot Sands. Many of the other outliers 

 of the Tertiaries in the neighbourhood of Newbury are also capped 

 with these gravels, as Snelsmore, Curridge, &c. South of the Kennet 

 they are found spread over the large commons of Greenham, Crook- 

 ham, the high ground about Enborne, and Newbury Wash. The 

 gravel covering the Lower Bagshot Beds at the north-west corner of 

 Inkpen Common is remarkable for the numerous blocks of Sarsen 

 stones which are irregularly scattered through it. At Pebble Hill 

 near Kintbury there are ten feet of rounded black pebbles imbedded 

 in yellow sand ; the extreme western outlier of the Tertiaries near 

 Shalbourn is also capped with pebble gravel. A similar formation 



