PRESTWICH WOOLWICH AND READING SERIES. 123 



patches of lignite, are occasionally met with in the lower part of the 

 mottled clays ; but they occur more commonly in association with 

 the fluviatile beds of Woolwich : they do not possess any importance, 

 although occasionally expanding in some places to the thickness of 

 a few feet, but are more often measureable by inches. The local 

 sections and details (§ 3.) give some of the main points of its oc- 

 currence *. 



It is at the base of this division that I should place the Websterite 

 and Hydrate of Alumina found by Dr. Mantell at Castle Hill, New- 

 haven. I have since found the latter with the pebbles and flints in 

 immediate contact with the chalk, at Northaw. 



Small crystals of sulphate of lime occur occasionally in the clays 

 and sands of the fluviatile group. A thin seam of clay found by 

 Mr. De la Condamine at Counter Hill, showed on the faces of lami- 

 nation a surface covered with very small sharp cavities, formed appa- 

 rently by crystals of selenite having been dissolved out. I have not 

 met with selenite in the mottled clays. 



A singular fact was noticed a few years since in the ' Annales des 

 Mines,' in connection with the mottled clays of the "Argile Plastique" 

 of the Paris basin, viz. that they contained a very considerable pro- 

 portion of gelatinous or soluble silica, ^. e. silica in an active chemical 

 state, and soluble in alkaline solutions without fusion-f . I have recently 

 tested the mottled clays from various places in the London district, 

 and find that they also contain this gelatinous silica, which can be 

 readily separated out by boiling in a solution of caustic potash. The 

 proportion, however, is very variable. This peculiar condition of the 

 silica has an important theoretical bearing. 



§ .5. Druid Sandstones. 



The position in the Tertiary series which should be assigned to 

 the large blocks of white saccharoid sandstone found scattered over 

 the surface of many parts of the chalk districts, and met with occa- 

 sionally within the tertiary area itself, has long been and still re- 

 mains an unsettled question. As these isolated blocks are always 

 siliceous, not unfrequently exhibit traces of rootlet-like impressions, 

 and occasionally contain round flint-pebbles and subangular slightly 

 worn flints, they present, with the latter exception, a lithological 

 structure very similar to that of the blocks found irregularly dispersed 

 sometimes in the lower, but more especially in the upper division of 

 the Bagshot Sands between Esher and Strathfieldsaye ; and, as no 

 other tertiary formation presents on first appearance so good a primd 

 facie right of possession, they were, in the absence of all distinct or- 



* It is too impure for use. At Lewisliara a heap of pyritous lignite which 

 had been thrown on one side, caught fire, and burnt for several months. It was 

 the occasional occuiTencc of lignite in these beds, here and at Blackheath, that 

 gave rise to the popular belief of coal beds existing at the latter place. 



t This was in 1846. The first notice of this peculiar condition of silica is, 

 r believe, that by M. Sauvage in 1840 (Ann. des Mines), when he detected the 

 presence of it in some of the beds of the lias and in some beds between the Chalk 

 and the Gault. 



