BEACHES, ETC., OF THE SOUTH OF EZJTGLAND. 



317 



(200 lbs. and upwards) varies in thickness from 1 to 20 feet, and 

 rests on an eroded bed of Palaeozoic and granitic rocks. 'No mam- 

 malian remains have been recorded from the workings ; but then 

 none have been found in the Head in this district, and the oppor- 

 tunities for search are rare. The Stream-tin drift is immediately 



Fig. 16. 





A. Alluvial beds with submerged Forest at base. 

 a. Erubble-drift (Stream-tin detritus). 

 c. Eaised Beach. 

 1. Substratum of rock. 



succeeded by alluvial beds containing only recent forms of life. It 

 will suffice to give an abstract of Colenso's well-known general 

 section, as the Stream-tin works all present similar features :-^ 



Section of the Stream-tin WorJcs in the Pentuan Valley} 



Eiver-sand and gravel 20 feet. 



Alluvial beds with recent marine shells 22f „ 



Forest-growth with stumps insitu 2 „ 



Stream- tin detritus 8(?),, 



The depth from the surface to the Stream-tin drift is here 

 54 feet ; but such a depth is unusual. It is more generally from 

 10 to 30 feet. The more usual depth to rock below the present high- 

 water mark may be taken at from 20 to 40 feet. 



The position of the Forest Bed in these sections corresponds with 

 that of the submerged forest at Porlock, in being above the angular 

 Eubble-drift — the equivalent of the Head, and therefore these sub- 

 merged forests are newer both than the Raised Beaches and the 

 Head. A different opinion is held by some geologists in conse- 

 quence of the teeth and bones of the Mammoth being found on the 

 ground of the submerged forest at Torbay."^ But this may be 

 accounted for by the circumstance that the sea-bed off shore must 

 frequently be formed, as at Porlock, of the angular rubble, and as 

 this, where the conditions are favourable, frequently contains 

 mammalian remains, their occurrence on the washed and denuded 

 submarine surface of this drift would be nothing extraordinary. 

 They have been found in the Head at Plymouth, and I have already 

 spoken of the occurrence of Horse at Hope's Nose. 



^ For details, see Colenso, Trans. Eoy. Greol. Sec. Cornwall, vol. iv. (1832) 

 p. 29, and De la Beche's ' Eeport on the Geology of Cornwall, &c.' p. 401. 



^ Grodwin-Austeu, Quart. Journ. (xeol. Soc. vol. vii. (1851) p. \61 ; and W. 

 PengeUy, ' On the Submerged Forests of Torbay.' 



