84 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



the tie-bars broke in succession, and on being replaced by new 

 ones, the timber gave way along a line about 20 feet below the 

 pile heads. Owing to this settlement, numerous cracks appeared 

 in the bank between the dam and the creek. This bank was 

 reduced, and the slope behind the dam tightened to as great an 

 extent as was deemed consistent with its efficiency. By this 

 means, and with the assistance of struts from the piles already 

 driven in the lock, the movement in the dam was so far arrested 

 as to allay the fear for the safety of the bank. 



An event occurred about this time which threw some light 

 on the origin of the eruptions. It has been already remarked 

 that the water from the boils was charged with yellow sand. 

 The only sand which had been found by boring or otherwise in 

 the lock pit was that composing the thin bed dividing the two 

 clays, and this was grey, loamy, and passing into small gravel in 

 places. A boring in the neighbourhood of the lock extending to 

 the surface of the chalk, which was reached at 109 feet below 

 the quay level, passed through a bed of sand overlaying that 

 formation. The great depth of this bed below the surface, and 

 the thickness of the overlaying clay, seemed to preclude the 

 possibility of the boils being in any way connected with it. To 

 ascertain the nature of the strata underlying the lock pit, a series 

 of borings were taken in three parallel lines along the centre line 

 of the lock and along the centre line of the walls ; the borings 

 were 25 feet apart, and extended to 65 feet below the coping. 

 Solid homogeneous brown clay was met with in every case for 

 the lower 10 feet. The borings were made with a common 



O 



auger, one inch in diameter, and without pipes, excepting in 

 places where the mud had not yet been removeJ from the 

 surface. At the fourth bore-hole from the piling at the lock 

 entrance the water boiled up charged with yellow sand, similar 

 to that brought up by the former boils. This occurrence seemed 

 to indicate that the boils had their source below the lower bed 

 of clay. To set the matter at rest, a 3-inch boring was made 

 through this bed ; the boring was began at a depth of 43 feet 

 below the coping, and a thickness of 42 feet of solid brown clay 

 was penetrated. This clay was free from water, and the bore- 

 hole remained dry until the bottom of the bed was reached, 

 when water charged with yellow sand flowed up the hole with 

 considerable force. The water was allowed to rise in a vertical 

 pipe connected with the bore-hole, and ceased to flow at a height 



