Vol. 68.] GLACIA.L OEIGIN OF THE CLAT-WITH-FLTNTS. 205 



3 to 4 miles wide running parallel with, and including, the present 

 valley of the Thames from Hurley to Bourne End, and continued 

 in the same east-north-easterly direction through Beaconsfield 

 and Chalfont to the Colne Yalley. South of this belt, between 

 Cookham and Uxbridge, the sandstone and quartzite pebbles are 

 much less abundant. 



Exceptionally, Bunter pebbles other than quartzites and sand- 

 stones are met with. Among the most remarkable rocks found are 

 a tourmaline (schorl)-breccia, an arkose, quartz-schist, and several 

 igneous rocks which appear to be rhyolites. 



At the southern margin there is some difficulty in separating 

 this gravel from the newer gravels — the terraces of the present 

 Thames and Colne. In some places, the boundary is a jjurely 

 arbitrary one ; but. as a rule, there is a distinct feature between 

 them. 



In tracing the gravel we are dependent, to a great extent, on 

 small sections a foot or two in depth and on ploughed fields. Of 

 more important sections we may mention one at The Hockett, near 

 Bisham, already described by Mr. H. W. Monckton,^ which yields 

 pebbles of white quartz, quartzite, sandstones of various colours, 

 quartz-grit, and pebbles of igneous rock, in addition to the usual 

 flints. 



In the angle between the Thames and the Wye, near Wooburn 

 Green, there is a platform in which is a brick-pit showing the 

 following section : — 



Thickness in feet. 



Grravelly wash , 1 



Brick-earth, with bauds of small angular flints 10 



Gravel, consisting of subangular flints and pebbles 6 



Although very careful search was made, no sandstone, quartzite, 

 or other far-travelled pebbles could be found in any part of the 

 section, except in the gravelly wash at the top. Behind the 

 platform, however, there is a steep rise, and in this a small gravel- 

 pit yielded many sandstone and quartzite pebbles and also a schorl- 

 breccia pebble. Between the two gravels is a narrow band of 

 yellow clay with a few flints (Clay-with-Flints), exposed in a pond- 

 section. 



About a mile due south of Chalfont St. Giles is a brickyard 

 showing 25 feet of yellow brick-earth, streaked with greyish bands 

 and current-bedded throughout. About 6 feet from the surface 

 occurs a thin, but fairly persistent, band of gravel composed of 

 flint-pebbles and subangular flints. At the bottom of the pit is 

 found a bed of whitish sand containing much clay. A small pit near 

 by, in gravel with the far -travelled pebbles, seems to be piped into 

 the brick-earbh. The latter may be correlated with the bedded 

 material at Walter's Ash. 



The last two sections are important, because they show that the 

 gravels with far-travelled pebbles are younger than the Clay-with- 

 Elints and associated Gravels. 



1 'On the Occurrence of Boulders & Pebbles from the Glacial Drift in 

 Gravels south of the Thames' Q. J. G. S. vol. xlix (1893) pp. 312-13. 



q2 



