305 



Ijorous rocks of the Great Oolite, lose much of their surface 

 character as developed by the Fuller's Earth, and pass down by- 

 percolation out of sight into absorbent beds in the Great 

 Oolite. As the fall of the river is less than the mesne dip of 

 these beds, they thus get under the impervious clays and rocks 

 of the Forest Marble (as at the low grounds at Malmesbury), 

 by which they are imprisoned until they find a fault in the 

 stratigraphic uniformity, up which they ascend as Artesian 

 springs, overflowing into the surface channels, and again 

 restored to sight. 



A similar peculiarity is remarkable in the river Chum, and 

 in a greater degree. That river, after passing impervious beds 

 of the Upper Lias for nearly 6 miles from its source, (at Seven 

 Wells) attains a volume of 300 to 400 cubic feet per minute at 

 Marisden Farm, whilst 6 miles lower down, after traversing the 

 porous beds and fractured freestones of the Inferior and Great 

 Oolites, on approaching Cirencester the volume becomes 

 diminished to 30 cubic feet per minute only, sho^ving a loss 

 of 320 cubic feet, or 2,000 gallons per minute. 



That the water is still under the valley has been demonstrated 

 by the Barton boring, carried out in 1872 at the expense of 

 Lord Bathuest, for I found and tapped it 91 feet below the 

 surface, to which it forcibly ascended through the bore and 

 overflowed. I do not, however, propose on this occasion to 

 enter on the large subject of the hydrology of our Cotteswolds, 

 on which I hope one day to be able to put together the notes 

 that I have been making for some years past ; but I take this 

 opportunity of calling the attention of the Club to the important 

 characteristic in the development of the streams of the district, 

 particularly of those flowing towards the Thames and the Avon, 

 to which I have referred. 



It is probable that the views tirged by our esteemed colleague. 

 Me. Witchell, in his interesting paper on the Denudation of 

 the Cotteswolds, as to the Formation of the Valleys being " the 

 result of subserial denudation including springs and rivers in 

 combination with landsHps," may receive some modification 

 from a consideration of the ** Circumstances of Flow " of the 



