18 
ing coast. The highest deposit consists of 20 feet of river sand and gravel 
the accumulation of a period during which the detritus brought down by 
the stream gained on the depression. The upper part of it, if not more, is 
most likely the result of ore-washing carried on for centuries higher up the 
valley. But at the bottom of the bed was found a row of piles probably 
intended for a foot bridge but their tops being only just on a level with 
low water mark they would be quite useless for that purpose in the present 
day. It was formerly supposed, and in this view Sir H. de la Beche con- 
curred in his Geological Survey of Devon and Cornwall, that the stanniferous 
gravel was the result of some violent and sudden flood that had rushed 
over the county. The direction of the flood was thought to have been 
*from N.W. to S.E. because the streams on the south were so much richer 
in stream-tin than those on the North. It is however on the principles of 
modern geology much more probable that the gravel is the result of the 
meteoric erosion of the land for a series of ages. 
Its greater abundance on the South Side of the county may readily be 
accounted for by the position of the watershed which lies much nearer to 
the Northern than the Southern Coast. The latter therefore would receive 
the washing of a very much larger extent of county. 
Further the Stream-tin occurs just where on this supposition it should 
occur— at the junction of two valleys especially if they meet at a high 
angle ; and at places where the slope of the valley becomes less and the 
current was consequently slower. Large quantities of stream-tin have been 
found inland in the Tregoss Moors a kind of basin-shaped depression 
through which the rapidity of the stream must have been less. In the St. 
Austell valley the deposit commences where the fall of the valley 
(iiminishes to about 45 feet in the mile or about 1 in 120. On this slope the 
heavy tin ore came to rest with of course a certain proportion of the lighter 
material the greater part of which however was carried further down by 
the force of the current. The process was in fact the same as that now 
adopted for washing the ore after it has been stamped at the mines. 
During this period the land stood about 50 or 60 feet higher than at present 
so that the rest of the lighter material was in all probability deposited on 
ground that now lies under the sea. On the tin thus deposited the oak 
trees above mentioned seem to have grown until a subsidence of the land 
set in which brought the sea water over the tin ground. This continued, 
sea and river wash alternately gaining on each other, until the forest or 
peat bed had reached the level at which it now lies about 50 feet below 
high water mark. 
