60 
ST. EHTH BEDS. 
Towards the south-east, where the ground rises, the deposits 
change very fast, for only 40 feet from where the last section was 
measured they were found to be as follows :— 
Feet. 
Head - - « - - - - 3 
Clean mottled clay - - - - - 7 
Loam with small quartz pebbles - - ~ % 
Sand - . . ^ . . . . 
Still higher, in the old sand pit in the adjoining field, the Head 
rests directly on sand, as it does also along the south-east and the 
greater part of the south-west face. Along the north-western 
face of the pit the clay seems to be most developed and continuous 
for we find at the north-west corner:—- 
Head ----- 
Brown clay - - - - 
Blue clay - - - - 
Gravel (not seen at the time of my visit). 
Further v/est the solid rock rises again, and el van is met with 
in a quarry close to Mellanear Lane, though a small overgrown pit 
nearer to the village seems to have been opened in gravel, perhaps 
the same gravel that underlies the fossiliferous clays. On the 
eastern side the deposit appears to abut against a cliff or steep 
bank, killas being visible at Carnabargas and again at Trenhayle. 
On the north the extent of the Pliocene Beds may be some¬ 
what greater than I ventured to lay down on the map which I 
prepared of the locality, on the scale of 25 inches to a mile, for 
some fields on the south-west side of Trelissick are flat, and 
apparently have a deep soil. However no sections were visible, 
and it is safer not to extend the outlier north of Mellanear Lane. 
The small outlier of marine beds at St. Erth probably owes its 
preservation to the position of the strata in a hollow or channel 
between a boss of slate and a hard projecting el van dyke. This 
part of the spur of Palaeozoic rocks happens also to be so isolated 
from the high land by small valleys, that no streams have been 
able to collect and to erode it; the Pliocene deposit can therefore 
suffer little from denudation, even under the heavy rainfall of 
Cornwall. 
As it seemed probable that other outliers might be preserved 
in a similar way, the whole valley was searched from Marazion to 
Lelant, but without result. Everywhere except near St. Erth the 
contours were so distinctly those of fluviatile denudation acting on 
hard strata that it was hopeless to expect anything but PalaBozoic 
rocks, more or less capped by Head. The only place where other 
similar outliers are likely to be found is on the watershed which 
lies about a mile to the west and south-west of the village of 
St. Erth. Here also is a ridge little affected by denudation, but 
unfortunately no sections were exposed; any pits, however, that 
may be opened in this area should be carefully examined. 
The exact age of the St. Erth clays still remains somewhat 
doubtful, and so many views have been expressed on this subject 
Feet. 
7 
4 
3 
