Bucklafid's Formation of the Valley of Khigsclere, 271 



We are a little at a loss to reconcile the latter paragraph 

 with the conclusion to which the author elsewhere arrives, in 

 conjunction with Dr. Buckland, that " the tertiary formations 

 were deposited in a horizontal position before the basins were 

 in existence." Admitting the justice of the conclusion, it is 

 not unreasonable to infer that the incumbent strata would 

 equally partake of the disturbances which so remarkably 

 affected the chalk. 



That the accident which impeded the progress of the tunnel 

 was occasioned by a fissure in the strata, of the description 

 alluded to, is, however, by no means apparent from the cir- 

 cumstances. The excavations were too near the surface to 

 afford sufficient proof that it was other than the usual " broken 

 ground" which forms the beds of most valleys. It is well 

 known, and observed in practical operations, such as forming 

 tunnels, driving levels, and sinking shafts, that all formations, 

 whether composed of soft or of indurated materials, are, to a 

 considerable depth below the surface, " shaken" and doubtful, 

 consequently treacherous. No mineral or coal beds " prove," 

 with any approach to certainty, until they have been pursued 

 beyond the influence of the operations which have disturbed 

 the exterior crust of the earth. We do not refer solely to the 

 irregular accumulations of what is called diluvial matter, but 

 to disturbances, diluvial or otherwise, which extend still deeper. 

 It is obvious that the thickness of diluvium or of transported 

 materials is, of all things, the most uncertain, particularly at 

 the bottoms of valleys, and in the beds of rivers. Nothing can 

 present a more ragged outline than a section of the base of a 

 diluvial district, or demonstrate more incontestibly that there 

 is not the slightest agreement between the outline of the base 

 on which the detritus is deposited, and the present contour of 

 the earth's surface. 



With regard to the geological position of the Thames tun- 

 nel, it has been customary to speak of its passing through the 

 London clay. If the excavation be pursued, it will probably 

 intersect that formation towards the northern portion of the 

 line ; but hitherto, from the materials which have been brought 

 up, it is quite evident that the work has proceeded in some of 

 the plastic clay beds, so denominated, agreeing with those 

 exposed at Woolwich, and much less favourably adapted to 

 subterraneous operations, than the tenacious mass of the Lon- 

 don clay which reposes upon it. The formations lying im- 

 mediately to the south-east of London, have by no means been 

 accurately defined upon our geological maps. 



T. 



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