36 CORNISH POST-TERTIARY GEOLOGY. 



crest of the bank being 18 feet above high-water mark, proving its 

 ^olian origin. Dr. Boase ' gives the following section of the bank : 



1. Granitic sand, of quartz, mica, and hornblende slates, with a 



little tin ore, quartz predominating 10ft. Oin. 



2. Gravel, pebbles of hornblende slates from 1 to 3 inches in 



diameter ICft. Oin. 



Eesting on the forest bed, which ranges from 12 to 20 feet 

 below high-water mark, and rests on decomposed slates. 



The West Green Sandbank is much shortened and greatly diminished 

 in area, there being evidence- to show that in Charles the Second's 

 time it afforded 36 acres of pasturage, whilst it is now but 2 or 

 3 acres in extent ; a diminution for which the large quantity of sand 

 abstracted for agricultural purposes ^ does not satisfactorily account. 



Gravel and Sand Bars are almost entirely confined to the south- 

 eastern coast-line. The Loo Bar consists of coarse sand and fine 

 quartz gravel, with occasional pebbles of slate and flint. In 1837 * 

 a boring was made in the middle of the Loo Bar to a depth of 68 

 feet, or 30 feet below low-water, without meeting rock. The details 

 of the boring are not given. The waters of the Loo Pool, at a 

 point 400 yards within the bar, are 40 feet in depth at their ordinary 

 level.' 



Swan Pool is dammed by a bar, chiefly composed of small quartz 

 pebble shingle, 80 yards in breadth, and from 3 to 5 feet above high- 

 water mark on its crest. 



At the mouth of the Pentuau Valley a bank of coarse granitic 

 sand, in alternate light and dark (schorlaceous) layers, showing false 

 bedding, and rising to a height of 5 feet above high-water mark, 

 separates the alluvial land (also superficially composed of granitic 

 sand) from the sea. A second ridge is formed by a low range of 

 sand dunes at 6 chains inland, probably the sun-dried drift from the 

 bank which dams the sea from the low-lying land at the mouth of 

 the valley. 



A low range of sand dunes runs across the head of the Par 

 Estuary, separating the alluvium from the sea-sands. Par Sands, 

 being of considerable extent, and uncovered for a long time between 

 the tides, would furnish the material for this ^a^olian drift. 



' Trans. Roy. Geol. Soc. Corn. vol. iii. p. 131. 



2 Ibid. vol. ii. p. 136, and vol. iii. p. 131. ^ Ibid. vol. vii. p. 31. 



* Mr. J. Rogers, ibid. vol. vii. p. 352. « UiU. 



