280 PROF. J. PRESTWICH ON THE RAISED 



and as a deep (above 100 feet) and, in all probability, old channel 

 intervenes, the Triassic materials in the lieach could have been 

 carried there only by floating ice. One boulder of a species of 

 trap rock derived from this source, and weighing ui)wards of 

 2 cwt., was found with others in the lower part of the Beach. 

 The shells belonged to the common varieties of Purpura, Patella^ 

 Buccinum, and Ostrea. Eones and teeth of Ox were found in the 

 overlying Head. At another spot, remains of the Mammoth and 

 Hyaena were found between the Head and the Eeach. Traces of 

 the Beach, covered by a Head of angular local debris, are to be seen 

 both on the western and eastern sides of Plymouth Sound. 



Between Plymouth and Falmouth a Eaised Beach exists at Looe 

 Island, Polkerris, Polmere Head, Spit Point, Pendower, Porthscatho, 

 and Perth (see Map, PL YIII.). 



There is a large amount of angular slate-debris, in places 30 

 to 40 feet thick, at the entrance to Falmouth Harbour and west 

 of Pendennis Castle. Mr. Whitley informs me that with it 

 there are intercalated " 5 beds of impalpable sand mixed with 

 crushed stones." In the lower part are some large blocks of the 

 local Killas. The Head extends a considerable distance seaward 

 from its base on the shore. The following section in descending 

 order was taken some years ago : — 



a. Angular debris of slate, with blocks of the same and veins of quartz, in a 



matrix of fine sand and loam. 

 h. Quartzose sand, with worn flat pieces or cakes of slate. 



c. Sand, with white quartz and slate-pebbles. 



d. Eaised Beach. 



As the Beach trends westward it is reduced to little more than 

 a bank of white quartz-pebbles, while with the decrease in height of 

 the shore the Head diminishes in thickness and passes into an 

 ordinary thin surface-drift, covering both the Beach and the low 

 intervening ridges of slate. In Swanpool Bay the Beach is from 

 6 to 8 feet above sea- level, and is apparently without shells. 



Thence to Penzance, portions of the Beach and Head are to be 

 seen near Bosemullion Head, at Ligwrath, Coverack Bay, Porthbeer, 

 Loo Pool, Pra Sands, and on the clifi's on the eastern side of Marazion 

 Bay. The noticeable points on this coast are the large proportion 

 of loam or earth in the Head, the large blocks of slate and Killas 

 that often occur in it, and the confused heaping of the fragments ; 

 also the large size of some of the slate- and granite-pebbles in the 

 Beach, which vary from the size of a hazel-nut to a foot in diameter : 

 and the presence of chalk-flints. The Beach is not unfrequently 

 concreted by iron peroxide or carbonate of lime into a solid mass, 

 often overhanging.^ 



From Penzance the Beach, hugs the coast round the Land's End 

 to St. Ives, showing that, notwithstanding the force of the Atlantic 

 waves, this promontory has undergone but little change of form 

 since the Eaised Beach period. Portions of the Beach and Head 



1 Ussher, Geol. Mag. for 1879, pp. 167-69. 



