66 PROCEEDINGS COTTESWOLD CLUB 1901 
probably belong to the Pszloceras planorbis zone, with 
portions of higher zones. Beds of P. planorbis age 
occur at Harptree Hill, resting upon the Old Red Sand- 
stone, Carboniferous Limestone, and “Dolomitic Con- 
glomerate.” Broadfield Down exhibits marginal deposits 
of Lower Lias age. These strata are best seen in the 
village of Downside above Brockley Combe. 
That the Lower Lias encroached upon the Mendip 
archipelago is proved by numerous dykes containing 
only the fauna of that period. In some parts of the 
archipelago subsidence seems to have been more rapid 
than in others. Clifton Island appears to have been sub- 
merged at this time, for a Liassic dyke is to be seen near 
the Suspension Bridge on the Gloucestershire side. 
In Middle and Upper Lias times, the Mendip sea was 
still very shallow. The base of the Middle Lias at Rad- 
stock is conglomeratic, and contains Echzoceras raricosta- 
tume and other shells, derived from lower zones, whilst it 
presents similar features nearer the Mendips. At Holwell, 
Middle Lias rests upon the Carboniferous Limestone. 
Upper Lias is present at Cranmore, as shown by 
Ammonites." 
Thus, though in many places Liassic strata rest 
directly upon Paleozoic rocks, there is no evidence, 
even in the form of dykes, of the complete sub- 
mergence of Mendip, Blackdown, or Quantock Islands. 
In late Liassic times, these three islands were 
probably all that was left of the Mendip Archipelago. 
Their area was greatly reduced, and the cliffs were 
probably steep. The mainland coastline was much 
further removed from the Mendip archipelago, as 
shown by the overlap of various strata on to the 
Palaeozoic rocks. The remarkable deposits of the Sutton 
1 S. S. Buckman, Monogr. Inf. Ool, Amm. (Pal. Soc.), p. 168, 1890. 
