﻿396 ME. G. BAEEOW OX THE [Aug. I908. 



These consist of peat, silt, and gravel, and form the overburden 

 that has to be removed before the claj- can be worked. But it has 

 already been pointed out that this clay (the ' shelf ' of the stream- 

 workers) is below the present water-level of the surrounding marsh, 

 and the difficulty is to keep this marsh-water out of the pit ; it is 

 essential to do so, as the water is peat-stained in wet weather, and 

 would render the clay of little value. So flat are these marshes in 

 cross-section, and so small is their fall in the direction of the water- 

 flow, that it would require a big drain nearly half a mile long to 

 obtain a fall sufficient to keep the water out without pumping. 



But in one case this difficulty does not occur ; this is at Stannon 

 Marsh, which lies above the 750-foot contour, and has sloping sides 

 instead of a flat waterlogged base. This departure from the type- 

 form is again connected with the 750 -foot platform. At the time 

 of its formation a bay-like hollow was formed in the lower part of 

 the valley in which the marsh lies ; and, on emergence from the sea, 

 this was left as a flat-based valley. AVater falling on the adjacent 

 slopes would accumulate at the edges of the valley and tend to set 

 up tAvo distinct streams. This clearly occurred, and the two can 

 still be traced ; the northernmost carried most water and was the 

 more deeply cut in its upper part, but both united before reaching 

 the Camel. 



Later on came the great post- Pliocene uplift, followed by a rapid 

 cutting-back and deepening of the bed of the Camel and its 

 tributaries. One of these, separated from the northernmost of the 

 two streams just described by quite a low divide, has succeeded in 

 breaching this divide and capturing the northern stream. 



The united streams have continued the cutting-back process, until 

 they have undermined the base and locally obliterated the southern- 

 most of the two valleys formed in the early stages of denudation of 

 the bay of the 750 -foot platform. The process, however, is so 

 recent that the stream flowing through the china-clay area is still 

 bordered by sloping banks, and does not yet lie in a flat-based 

 marsh : such a marsh will be developed in due course, as in the 

 case of all the other streams traversing china-clay, but at present 

 the cycle is incomplete. On the sloping bank left by incomplete 

 denudation, the clay-pit of Stannon Marsh is situated, and the 

 great trouble with peat-stained surface-water is avoided. 



It may be worth while here to note a curious observation made 

 by Dr. Tempest Anderson in St. Vincent (West Indies). The great 

 outburst of ashes that took place recently levelled the base of a 

 river-valley so as to make it flat in cross-section, and exactly the 

 same result has been produced as in the case here quoted in 

 connexion with the bay of the 750-foot platform ; namely, two 

 streams have been set up, one at each margin of the choked and 

 partly-levelled valley. 



