XCIV PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 1914, 



years ago, on the scale of 10 miles to the inch. The areas of the 

 basins are given, but they include the ground which drains into 

 tidal water, whereas for our purpose it was necessary to exclude 

 that part of each basin. 



In practice, the precise determination of a water-parting proved 

 to be far from easy. For example, where a plateau, such as that 

 which dominates the landscape in the south-eastern part of Devon, 

 has been dissected by a system of streams, the water-partings run 

 along surviving strips of the plateau, which may range from a few 

 yards to a mile or two in width. The strips are flat-topped, and 

 their drainage is usually artificially effected and liable to be 

 changed. In the case mentioned much of the plateau is formed 

 by gravel, which rests on an inclined plane of impervious strata. 

 The water which soaks into the ground breaks out as springs on 

 the side towards which the plane inclines, and is far from being 

 equally divided between the drainage-systems on either side of the 

 plateau. There is much ground also where, for other reasons, no 

 definite line can be drawn, and in all such cases it is better to adopt 

 an arbitrary line than to aspire to an impossible precision. All 

 this notwithstanding, a river-basin map of sufficient precision 

 for practical purposes is possible, and it might well be made both 

 for academic and for administrative reasons. 



The main water-parting once determined, it became possible to 

 make such other measurements within the basin as were desirable : 

 such, for example, as the areas lying above 1000 feet, or between 

 1000 and 500 feet above the sea. 



Lastly, the character of the strata cropping out within each 

 river-basin was considered. Unfortunately the greater part of 

 ^ach of the three river-basins lay outside the area to which the 

 mapping of the Drift and the revision of the solid geology by 

 the Geological Survey has extended. It was necessar} r , therefore, 

 to have recourse to the Old Series G-eological Maps, and to 

 generalize somewhat broadly on the distribution of Drift, and on 

 the relative areas occupied by pervious or impervious subsoils. 



The geological results of the investigation, with which alone we 

 are concerned in this Society, were fairly in accordance with 

 expectation. The amount of suspended matter was not measur- 

 able, or was extremely small, when the current was normal or below- 

 normal. It increased rapidly when the river, in rising, reached a 

 certain level ; but the rise in the amount was not always in direct 

 relation to the rise in the water-level. We had invariably before us 



