66 
ST. ERTH BEDS. 
absence of truly littoral forms is due to the distance from the 
shore. 
During the examination of this part of Cornwall I was struck' 
by the rounded hummocky contours of the lower portions, contours 
that did not seem to be due to fluviatile action, for in hard rocks 
like the slates and granites of Cornwall subaerial denudation tends 
to develop bold scarps rather than flowing curves. This smoothed 
character appeared, however, to be exactly what one might expect 
if the land had been submerged to the extent of 300 or 400 feet, 
and everything below that level had been planed and smoothed by 
the Atlantic waves. Such a submergence ought to have left traces 
in ancient beaches; we accordingly find at St. Agnes Beacon, 
which rises to 627 feet, a series of clays and sands at exactly the 
level needed, z.e., between 800 and 400 feet. 
The sands and clays of St. Agnes Beacon have long been known, 
and as far back as 1839 they were referred by De la Bechet to 
the Tertiary period, and pointed to as more ancient than the 
oldest of the superficial deposits. These strata nearly encircle the 
higher part of the Beacon, reaching, according to Dr. Boase, a 
height of 375 feet above high-water level. They have up till now 
proved entirely unfossiliferous, but their position is so unlike that 
of any of the raised beaches, and they correspond so well in level 
with the height at which we might expect to find littoral deposits 
of the age of the St. Erth clays, that it is safe provisionally to 
refer them to the same period. 
The following section, observed by De la Beche on the north¬ 
east side of the hill, is deeper than any now visible :— 
St Agnes Beacon — north-^east side. 
Feet. 
Head of rubble, derived from the hill above, and 
named GoUb - - - - - 3 
Yellow sand - - - - - - 2 
Brownish sand, with numerous planes (apparently 
those of deposit) dipping at an angle of 45° - 11 
Light-coloured mining clay - - - - 2 
Blue clay - - - - - - 9 
Yellow sand - - - - - - 4 
White sand - - - - - - 4 
Yellow sand - - - - - - 3 
Pebbles, resting upon an uneven surface of slate variable. 
The strata vary greatly within short distances, and in the 
sections now exposed they are stained and mottled in such a way 
as to suggest the action of percolating rain-water. Any fossils that 
were originally contained in these sands and clays have probably 
been entirely dissolved out, for the beds seem to be nearly free 
from lime. No section is now visible of the thick bed of blue 
* Hawkins, John. On a very singular Deposit of Alluvial Matter on St. Agnes 
Beacon. T'rans. Geol. Soc. Cornwall, vol. iv. pp. 135-144. (1832.) 
Boase, Dr. H. S. Contributions towards a knowledge of the Geology of Cornwall, 
ibid. pp. 296-299. (1832.) 
t Report on the Geology of Cornwall, Devon, and West Somerset. 8vo. pp. 
258-260. (1839.) 
