TERTIARY STRATA. 
113 
snarls consist of various deposites, with sand abound- 
ing in fossil shells, of which about forty per cent, 
belong to existing species. The tertiary beds of 
Sicily consist of stratified marine deposites of clay, 
sand, and hmestone, at great heights above the sea, 
and which contain 95 per cent, of existing species. 
The following section illustrates the tertiary for- 
mations in the valley of the Thames (Eng.). 
1. Marine sand. 2. London clay. 3. Plastic clay and sand. 
4. Chalk. 5. Green sand. 6. River Thames. 
Bakewell observes, that from this small section 
the geological student may form some idea of the 
devastating effects of mighty inundations, which 
have swept over the surface of the globe, and car- 
ried away considerable portions of the upper beds. 
The marine sand, 1 1, which forms isolated caps on 
several of the hills in the Vale of the Thames, was 
probably part of one continuous bed, which has 
been excavated with a portion of the subjacent Lon- 
don clay; such excavations are common phenom- 
ena in almost every country. 
The following table will exhibit, at one view, the 
average thickness, in England, of the formations 
we have now described 
Fig. 31. 
Feet. 
80 
100 
600 
K2 
