﻿Vol. 64.] HIGH-LEVEL TLATFOEMS OF BODMIN MOOE. 393 



with a sheet of peat, varying in thickness from 1 to as much as 

 15 feet, and in rare cases even more. Below the peat is the recent 

 wash, a very fine deposit carrying no tin and also variable in 

 thickness ; this is always greatest where large streams flow through 

 the marsh, and it is never thick where the stream is small. Below 

 this is the tin-bearing gravelly deposit, formed during the cold 

 period, w^hen all the finest material was swept entirely away, 

 leaving the tin somewhat concentrated in what is really the 

 assorted wash swept down from the hill-sides. In working this 

 deposit, it was found that in all cases the richest part lay in a 

 distinct channel below the general level of the shelf or granite- 

 floor on which the rest of the deposit lay ; in this channel the 

 gravel was thickest, and the tin most concentrated, but in many 

 cases its course diff'ers markedly from that of the stream at present 

 flowing through these marshes. These old channels were steadily 

 followed up by the streamers, who also worked the less rich gravel 

 on either side as far as possible, or until it no longer paid to do so. 

 It is generally supposed that most of the patches left in the 

 lateral gravels were not worked because they were too poor, but 

 it is most probable that the true cause in many cases is that they 

 were waterlogged. 



This waterlogging is due to the great change in climatic con- 

 ditions : during the cold period the rush of water was great enough 

 to prevent the deposition of any material above the basal gravel ; 

 but, as the rainfall greatly decreased, this was no longer the case 

 and a process of infilling of these flats then followed. It began 

 with the deposition of the recent very fine wash and culminated 

 with the thick peat-growth. More than this, the base of the more 

 recent stream was steadily raised, and now always stands some feet 

 (at times many feet) above the base of the hollow or channel in 

 which the stream-tin is most abundant. 



Thus, unless they resorted to pumping, which they often did, 

 it was impossible to start work within the marsh itself; the 

 surrounding water-level was considerably above the work, and it 

 was consequently necessary to start some way down the stream 

 below the marsh, where the fall was greater. In many cases an 

 artificial channel had to be made, to carry off the water of the main 

 stream ; but this difiiculty was avoided when the old channel was a 

 considerable distance from the existing higher channel. 



The system of working was and is distinctly primitive, the field 

 of operations covering only a small area at a time, which tends to a 

 short-sighted policy in matters that may be of great consequence in 

 the future. This was specially shown in the disposal of the refuse 

 of the washings from the tie or tye, which was in most cases 

 dumped down in heaps behind the working-face and tended to 

 obstruct the escape of the water from the tye, so that the working- 

 face of the deposit became waterlogged. 



There seems good reason to believe that by this process many 

 patches became waterlogged, and owing to this the streamers locally 

 resorted to pumping to win the tin of the deeper channels. On the 



