TERTIARY STRATA. 



113 



marls 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 limestone, 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 



Erobably part of one continuous bed, which has 

 een 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. 



( Pliocene 



80 

 100 

 600 



780 



