ST. ERTH BEDS. 
65 
the same series, rise about 30 feet higher near St. Erth Vicarage 
The land rises rapidly to the south-east, so that we find an 
elevation of 255 feet only 600 yards away, and it should not 
be forgotten that the land was once still higher, for since 
Pliocene times this hill has “yielded the mass of angular rubble 
which now covers the fossiliferous bed. If the depth at which 
the clay was deposited were only 15 fathoms this elevation must 
then have been dry land, in fact the shore line must have coin¬ 
cided with the 190 foot contour, only 400 yards away. What 
evidence is there of this close proximity of land ? The httoral 
shells, such as Patella and Littorina, which everywhere occur in 
thousands on rocky shores between tide-marks, are entirely 
absent, as are Scrobicularia, Cardium edule, and Hydrohia ulvm, 
which inhabit the sheltered creeks and estuaries. The only 
evidence of the neighbourhood of land is the finding of a specimen 
of the littoral air-breathing Conovidus {Melampus) pyramidalis ; 
the empty shells of recent species of this genus will, however, 
float long distances when full of air. 
Again the lithological character of the clay points to deposit in 
fairly still water ; but as this area would be exposed to the force of 
the Atlantic swell, though somewhat sheltered by the high lands 
to the west, it is difficult to conceive of such clays being laid down 
in a less depth than 40 fathoms, especially when we take into 
account the enormous force of the waves at St. Ives and Penzance, 
which are now as much sheltered from the prevalent wind as 
St. Erth could have been. 
Assuming that the clays were deposited in 40 fathoms, the total 
depression of this area must have amounted to about 340 feet, for 
the fossiliferous deposit now lies about 100 feet above the sea- 
level. Under such conditions the character of the district would 
be greatly altered. Cornwall would be transformed into a scattered 
archipelago, divided by numerous straits and channels with very 
uneven bottoms, exactly as we find among the Scilly Islands at 
the present day. On this assumption the high land close to 
St. Erth would become a submerged shoal, reaching the lower 
part of the-laminarian zone, but not rising above the sea-level. 
Like all such rocky shoals it would be covered with a thick 
growth of the sea-weeds which form ihe favourite haunts of the 
various plant-eating liissoas and Odostomias. Anyone who has 
collected floating sea-weeds will have noticed how largely they 
assist in the transportation of these shells. Thus a deep hollow 
close to a forest of oarweeds must often be filled with the shells 
of the laminarian zone to such an extent as almost entirely to 
mask the true character of the fauna proper to the depression. 
Besides this the submarine slope was here so steep that multitudes 
of shells must have been washed down by the to-and-fro motion of 
the water and dropped into any sheltered hollow below the range 
of wave action. It is in this manner that I ihink we can account 
for the character of the fauna of the St. Erth clay ; the mollusca 
are mainly the species that lived on the shoal close by, and the 
E 60798. E 
