ON THE CIRCULATION OF UNDERGROUND WATERS. 215 



In reference to the information furnished by Mr. Taunton as to the 

 Thames and Severn Canal, it may be well to state that the outcrop of the 

 oolitic rocks has an average breadth on the dip of 25 miles. The base of 

 the Oolites resting on the Lias reaches its highest point near Chipping 

 Campden, 1,032 feet above the sea, on the watershed between the Thames 

 and Severn basins. This, south of the Seven Wells, the source of the 

 Churn, runs somewhat east of the base of the Oolite, causing the surface 

 drainage of the oolitic tract around Minchinhampton, Dursley, and 

 Wotton-under-Edge to flow into the basin of the Severn. It is probable 

 also that a portion of the underground drainage does so also, notwith- 

 standing the general south-easterly dip, from the basement level of the 

 Oolites, varying in the direction of the strike, owing to the denudation 

 of the escarpment being unequal, the Oolite to the south having been 

 worn back much further down the dip, and consequently to a lower 

 elevation than at Chipping Campden, descending from 1,030 at the 

 latter place to 212 feet in the Stroud valley, or about 800 feet in 25 miles. 

 South of this valley the level rises slightly, so that a partial discharge of 

 underground drainage takes place in this valley, which is immediately 

 west of the point in the Thames and Severn vratershed which is pene- 

 trated by the canal connecting the two basins. 



Of the 25 miles of average outcrop of oolitic rocks measured on the 

 dip, only about 8 consist of impermeable deposits — viz., the Fuller's Earth, 

 the Oxford Clay, and the Kimmeridge Clay, — so that two-thirds of the area 

 may be considered to be of a permeable character. The vertical section 

 of the Oolites is as follows : — 



Portland Oolite — 



Kimmeridge Clay ....... — 



Coralline Oolite ........ — 



Oxford Clay 300 



Cornhrash 8 



Forest Blarhle 30 



Great Oolite 200 



Inferior Oolite ........ 264 



WarwicJcsliire information. — The southern and western portion of the 

 Warwickshire coalfield is overlaid by Permian rocks consisting of reddish- 

 brown and purple sandstones, intercalated with marls in lenticular beds, 

 rising to a height of 622 feet at Cowley Hall, which forms part of the 

 watershed between the tributaries of the Trent to the north, and those of 

 the Avon on the south. 



Though the surface-drainage of this Permian area flows in opposite 

 directions, that portion of the rainfall percolating into the ground has a 

 uniform gradient to the south, the base of the Permians, where they rest 

 on the coal measures west of Atherstone, being 470 feet above the sea, 

 and 170 feet under the Mithurst Tunnel of the Midland Railway, being a 

 fall of 50 feet per mile, while at Warwick the tops of the Permians are 

 18C feet above the sea, and as they are not less than 800 feet thick, their 

 base is probably about 600 feet below the sea-level, giving a further fall 

 of 786 feet in 18 miles, or a fall of 4.3 feet per mile. 



Examining the district more minutely, it is seen that though the 

 Permians do not always lie conformably on the coal measures, yet 

 there is a general conformity, and a synclinal flexure traversing the 

 coal measures from north to south is shared by the overlying Permians, 

 which have synclinal dips towards the axis of an average amount of 3°, 



