produced in the strata beneath an embankment. 5^5 



of gravel varying from ten to three feet in thickness, hut which thins 

 out in some places, and under it is the regular London clay, traversed 

 in almost every direction by slimy joints. The surface of the country 

 gradually slopes towards the Brent, the difference of level between the 

 south side of the embankment and the Brent being about twenty feet. 



On the night of the 21st of May 1837 the embankment began to 

 settle, and in the morning it was found that the foundation had given 

 way, and that on the south side, or towards the Brent, a mass of 

 ground, fifty feet long and fifteen feet wide, had protruded from under 

 the earthwork. During the four succeeding months this mass con- 

 tinued to increase in dimensions, and the disturbance to extend, so 

 that the surface, for a considerable distance from the base of the 

 embankment, had assumed an undulated outline, and the subjacent 

 beds, where cut into, exhibited corresponding curvatures, overlappings 

 and cracks, the whole of which are described in the memoir, but can- 

 not be rendered intelligible without diagrams. In the embankment 

 itself the symptoms of failure were confined to a settlement of about 

 fifteen feet, and a large fissure near the top, on the side opposite to 

 that where the foundation had yielded, and which extended the whole 

 length of the slip. To this fissure, and its dip towards the disturb- 

 ance at the base of the embankment, the author particularly directs 

 attention, as he infers from it the nature and inclination of a fault 

 exhibited in the diagrams which illustrate the memoir. 



At the end of twelve additional months, during which the embank- 

 ment continued to slip, and the disturbance at the base to increase, 

 Mr. Brunei directed a supplementary earthwork or terrace to be 

 thrown down upon the swollen surface, and it was an effectual re- 

 medy. Up to this time the total subsidence had exceeded thirty 

 feet ; and the swollen ground, which extended nearly 400 feet in 

 length, and from seventy to eighty feet in width, had attained an 

 average height of ten feet, with a horizontal motion of fifteen feet ; 

 but the general disturbance ranged to a distance of 220 feet from 

 the foot of the slope, or to the Brent, the bank of which was forced 

 five feet forwards : the faults varied from thirty feet to two feet, and 

 the contortions had attained a curvature, the semi- axis of which was 

 in many places eight feet. 



The author then dwells on the magnitude of the disturbance, and 

 on the effects which may have been produced in the strata com- 

 posing the earth's surface, by pressure from above. He says, that in 

 consequence of the great inequality in the thickness of the sedimen- 

 tary rocks, due to the conditions under which they were deposited, 

 great inequality of pressure must have arisen, and consequently con- 

 tortions and faults have been produced, varying in amount according 

 to the thickness and the degree of consolidation in the strata them- 

 selves. In support of his argument, the author quotes a passage 

 contained in Mr. Greenough's 'Critical Examination of the Principles 

 of Geology,' and which asks the question whether contortions may 

 not have taken place where clay alternates with limestone or silex, 

 in consequence of an unequal rate of consolidation (p. 77). The 

 author also alludes to the theory of Sir James Hall, but chiefly to 



Phil, Mag. S. 3. No. U 1 , SuppU Vol. 2 1 . 2 O 



