part 1] DEPOSITS AT WORMS HEATH. 25 



Pebbles and Conglomerate. 



The pebbles consist almost wholly of flint. They are well 

 rounded and more or less graded, and frequently show shallow 

 white depressions where decomposition seems to have begun at the- 

 points of contact with other pebbles. Very occasionally a flattened 

 brown pebble of quartzite may be seen, such as occur sparsely 

 among the Blackheath pebbles of the Addington Hills, Hayes 

 Common, and other localities. 1 The space between the pebbles is- 

 occupied by smaller, less rounded, or broken stones, and fine loamy 

 sand like that just described. This matrix is generally iron-stained 

 to a vivid red or yellow, except where the iron has been leached 

 out through the reducing action of vegetation. 



As in the case of the sand, there is much ferruginous cementation,, 

 forming blocks of conglomerate which are used locally for orna- 

 menting gardens, and even carted to Croydon and elsewhere. Some 

 of these blocks are very hard and tough, breaking through the 

 pebbles as often as round them. There seems, however, to be no 

 silicification. Gleaming flakes of mica often lie close to the 

 pebbles, and the quartz-grains sometimes show gleaming surfaces,, 

 though secondary growth cannot be shown to have taken place. 

 Some of the pebbles are coated with finely botryoidal black gothite, 

 which may show a purple, blue, or yellow tarnish, or with amorphous 

 limonite. Hollow spheres of ferruginous matter, 0*5 to 1*0 mm... 

 in diameter, are seen in some specimens. 



Halloysite. 



A white clay-like material found in the pipes was at first regarded 

 as allophane, although the absence of the characteristic waxy lustre 

 and the low water-content suggested that it was more probably 

 hahVysite. 2 Chemical evidence has since confirmed this. It con- 

 sists of white opaque material, ironstained along cracks, and 

 often full of sand-grains, though pieces free from sand are also 

 present. It appears to occur in the sand in the pipes, sometimes 

 near the junction with the pebbles, sometimes near the Clay-with- 

 Flints, but the association with a green-coated flint partly covered 

 with the material indicates that it may have originated in the 

 Bullhead Bed at the base of the Eocene, which is a well-known 

 horizon for allophane and allied minerals. 



The material is traversed by shrinkage-cracks, especially where 

 free from sand, and, although it crumbles and softens when wetted, 

 it does not go down to mud. Thin sections ma} r thus be easily 

 prepared. The warm Canada balsam, however, turns the material 

 red, and other organic media give it red, purple, or blue colours at 

 temperatures below 100° C, while ignition only intensities the 

 coloration. 



1 Mr. A. L. Leach, F.G.S., has also found in the principal workings at 

 Worms Heath a pebble of dark banded quartzite. 



- G. M. Davies, ' The Rocks & Minerals of the Croydon Regional Survey 

 Area' Trans. Croydon Nat. Hist. & Sci. Soc. (1915-16) p. 92. 



