510 H. H. Uoicorth — Traces of a Great Pod-Glacial Flood. 



and exposed to the action of tides and currents transporting detritus, 

 we should expect, in accordance with the effects now produced by 

 such agents, that they would become more or less filled up by drifted 

 substances ; and, if they were valleys above the level of the sea, as 

 they now are, that the usual drift of transported matter would have 

 partially covered their lower parts in the manner we now see. " If," 

 he adds, " the stream-tin occurred in frequent layers, and was irregu- 

 larly dispersed in the mass of detritus now found in the valleys 

 containing it, it might be supposed that rivers, running over ground 

 traversed by strings or lodes of tin, brought down pebbles of tin ore 

 when in flood, distributing them here and there according to circum- 

 stances, while at other times they merely carried forwards the ordi- 

 nary and less ponderous detritus. So far from the tin-stones being 

 thus dispersed through the mass of gravels and sands in the valleys 

 containing them, we find the pebbles at the base of the whole, 

 resting upon the subjacent rock, commonly termed the shelf, forcing 

 their way, particularly when the tin-stone grains are fine, into all the 



chinks and crevices on its surface The stream-tin is always, 



moreover, found in greater abundance where we may suppose that 

 eddies would be produced in a body of water pouring down the 

 stanniferous valleys, these having the general foi^ms they now possess. 

 It appears as if any previous detrital contents of the stanniferous 

 valleys, should such contents have existed, had been fairly washed 

 out by a mass of water rushing over the land, rolling and driving 

 various loose materials hefore it, and allowing the tin-stone from 

 its greater specific gravity to be strewed along the bottom, where 

 circumstances permitted. If we regard the surface of Cornwall, 

 on which a crust of decomposed or disintegrated rock now exists, 

 one arising from the action of atn5ospheric influences, and imagine 

 a body of loater to rush violently over it, carrying this disintegrated or 

 decomposed surface hefore it doion the present valleys, stream-tin would 

 be found distributed in them much as it has been hitherto discovered. 

 .... It will be readily seen that the tendency of the tin-stones to 

 come to rest, from diminished velocity in water capable of trans- 

 porting all these substances, would be far greater than that of the 

 other mineral bodies usually occurring as gravel with it, all other 

 circumstances being equal. "When, therefore, a transporting water 

 could no longer carry tin-stones onward, it would be capable of 

 pushing forward or retaining in mechanical suspension the other 

 gravel associated with it ; so that a fundamental layer of tin-stone 

 pebbles might be accumulated at the bottom of a valley, and remain 

 settled, while lighter bodies were driven or carried over it, due 

 allowance being made for the forms and volumes of the component 

 parts of the whole transported detritus, tin-stone pebbles included. 

 Though Mr. Carne some time since called attention to the general 

 facts connected with the stream-tin of Cornwall as furnishing strong 

 evidence of a sweeping inundation having passed over the land, 

 ' the efi"ects produced by which have never been repeated by any 

 subsequent flood,' less attention seems to have been paid to this 

 evidence, while the distributing causes of various superficial gravels 



