BRITISH POSTGLACIAL & RECENT DEPOSITS. 439 



8. Roots of trees in their natural position ; oaks with 



fibres traceable for 2 feet deep. From the 

 manner in which they spread there can be no 

 doubt but that the trees have grown and fallen 

 on the spots where their roots are found. Oysters 

 still remain fastened to some of the larger stones 

 and to the stumps of trees. 



9. Tin-ground, with rounded pieces of granite, and sub- 



angular pieces of slate and greenstone. Most of 

 the tin occurs in the lower part, from the size of 

 the finest sand to pebbles 1 lbs. in weight ; some 

 rocks highly impregnated with tin weigh 200 lbs. 

 and upwards. Thickness (including No. 8) from 3 ft. to 10 ft. 



This section occurs near the sea ; when the tin-ground is 

 followed inland, the marine deposits disappear and the stanni- 

 ferous gravel is covered directly by freshwater accumulations. 

 It must not be supposed that all the tin-workings in the neigh- 

 bourhood of the sea exhibit precisely the same succession of 

 deposits as that given in the above section, but they agree 

 generally in showing that the tin-ground is overlaid directly by 

 an old forest -bed, and that this bed is in turn buried under 

 marine, estuarine, and fluviatile deposits. Besting upon these 

 overlying deposits, and sometimes intercalated with them, occur 

 occasional seams of vegetable matter, and now and again a well- 

 defined bed of peat with roots of trees occurs near the top of 

 the series, as in the following section : — 



Section of Lower Pentewan Work, \ mile from sea-beach. 1 



1. Soil with growing trees, some very old ; gravelly towards Feet. 



the bottom ........ 3 



2. Fine peat, roots of trees, fallen trunks, sticks, ivy, sea laver, 



rushes, impregnated with salt . . . . .12 



3. Sea-mud, with compressed leaves at the top, cockles at 31 



feet from the surface, bones, human skulls (one of a child), 

 deer horns. At the bottom a bed of very small shells, a 

 foot in thickness . . . . . , .20 



4. Sea-mud, oysters and cockles ..... 4 



5. Compressed leaves, vegetable matter, a few rotten shells . 6|- 



1 Trans. Royal Geol. Soc. Cornwall, vol. iv. p. 404. 



