TIIK M|.n;ocKNK SKIMKS 5G9 



n. Dining tin- Hoc. MI. peri<xl all the north-western part of 

 our i.^'imi I'm-med ;i trad of high plateau-land mi which a 

 remarkable series of volcanic eruptions took place, as briefly 

 described mi pp. 544 et seq. The southern part of this western 

 platrau is believed to liavt- drained eastward or south-eastward across 

 Knirland, luit after the cessation of volcanic activity crust-move- 

 ments occurred, whirli resulted in the elevation of the Pennine 

 range and the dome of the Lake District Dr. Marr 27 has 

 shown that the final uplifts of both of these areas took place in 

 Tertiary times, and I have given reasons for assigning the date to 

 late Eocene or early Oligocene time. 28 



It is probable that these uplifts were correlative with the 

 Mib.-idcnces which Conned the trough of the Hebridean Sea west of 

 Scotland, and that of the Irish Channel between Ireland and 

 Wales ; these depressions then becoming broad valleys which 

 drained southward, and thus formed new features in the geography 

 of the British region. Moreover, the Pennine uplift was probably 

 continued southward down the centre of England, forming a 

 watershed which threw off streams to the east and the west, and 

 thus initiating the valleys of the Severn and the Thames. 



Recurring now to the area in which the varied succession of 

 Eocene and Oligocene deposits took place, some remarks may be 

 made on the source of the materials of which they consist Of 

 course, the sands and clays were carried down by the rivers which 

 drained into the seas from the surrounding land, and the marine 

 limestones are mainly of organic origin. Two kinds of sediment, 

 however, are especially conspicuous ; the one being clays of white, 

 green, or variegated red and white tints, the other consisting of 

 pale-coloured fnrainiuiferal limestones. 



The clays occur on many horizons, and are generally of a 

 plastic nature ; in England they are found in the Reading Beds, 

 iu the Bagshot sands, in the Bournemouth facies of the Brack- 

 lesham Ileds, while in the Headon and Bembridge Beds green 

 clays predominate, though white, red, and variegated clays are also 

 of frequent occurrence. Similar clays occur at successive horizons 

 in the deposits of the Paris basin. 



The white clays have engaged the attention of several observers, 

 the earliest writers attributing them to the detrition of granitic 

 rocks and the decomposition of felspars. Mr. G. Maw, 29 however, in 

 1867 dissented from this view, regarding them as more probably 

 derived from argillaceous chalks ; Dr. A. Irving 30 also attributed 

 the Bagshot pipeclays to the "decomposition of chalk and the 

 leaching out of the iron by the action of peaty acids.'' 



There are, however, strong objections to both these views ; to 



