210 PETRIFACTIONS AND THEIR TEACHINGS. CHAP. III. 



of superficial transported materials, are omitted in the follow- 

 ing commentary. 



As each of these systems or formations is many hundred 

 feet in thickness, it follows that the inferior strata would have 

 been for ever concealed from observation, had the original 

 order of superposition of the respective deposits remained 

 undisturbed. But not only portions of each of the upper 

 series, but even of the lowermost deposits, appear on the sur- 

 face, and form an extensive tract of country, diversified by 

 hills and valleys, and constituting one of the richest agricul- 

 tural provinces of the British Isles. This distribution of the 

 strata has originated from the displacements occasioned by 

 those disturbing forces which have been in activity from the 

 earliest periods of the present physical condition of the earth, 

 and by which the beds of the ancient oceans and rivers have 

 been elevated above the water, and converted into dry land. 

 The nature of those changes we will now consider. 



The Eocene deposits form the area on which London is 

 situated r and extend over the surrounding country to a dis- 

 tance varying from ten to twenty or thirty miles. Around 

 the margin of this series of strata the Chalk appears, and 

 forms a distinct chain of hills on the south, w T est, and north ; 

 but on the east the range is broken by the valley through 

 which the Thames winds its way to the sea. From this geo- 

 logical character of the metropolitan district, it results that 

 all the lines of railway that proceed from London, traverse 

 for the first ten or twenty miles beds of tertiary clay, loam, and 

 loosely aggregated sand and gravel ; hence the numerous slips 

 that have taken place in the embankments at New Cross, 

 Wandsworth, Hanwell, &c. ; and in all these lines it is ob- 

 vious from the nature of the soil and the steepness of the 

 cuttings, that similar subsidences will again occur. 



The next geological feature observable along the London 

 railways is the Chalk, which is intersected by steep cuttings 

 and tunnels ; as for example in the South-eastern line, from 

 Croydon to Merstham ; and in the Great Western, from 

 Maidenhead to beyond Wallingford. After passing through 

 the Chalk, the lower subdivisions of the Cretaceous forma- 

 tion, viz. the Marl, Firestone, Gait, and Greensand, successively 

 appear ; and these are followed by the Oolite on the Great 

 Western and Birmingham lines, and by the Wealden on the 



