GEOLOGY APPLIED TO CIVIL ENGINEERING. US 



the comparative extent denoted l^y the triangle F. G. H. 

 The obvious reasons of this occurrence must therefore have 

 been a defect in the security of the mass on the Une A. B, 

 and the first displacement was that of the mass of earth, 

 the sectional extent of which is represented by the letters 

 G. F. H, forming a portion of the larger triangle A. B. E, 

 situated between the line of dip and that of the slope 

 of the cutting. Under ordinary circumstances this slip 

 would have extended to the whole triangle A. B. 

 but the cubic quantity of earth displaced, and precipitated 

 below, having added to the security of the subjacent portion 

 A. G. H, the slip was confined to the upper portion, as 

 stated. We have thus endeavoured to explain the invariable 

 rules of nature, in subverting the most careful mechanical 

 calculations ; for under ordinary circumstances, no greater 

 security could have been required beyond tliat which the 

 angle of the slope presented. We have shown how pro- 

 minently the atmospheric changes influence the security, 

 independent of any internal disintegrating causes; but in 

 this particular instance, the composition of the clay itself 

 offers additional illustration. We are indebted to R. S. 

 Young, Esq., the Secretary of the Croydon Railway, for the 

 following extract from the report on the causes of the slip, 

 prepared some time since for the Board of Directors, and as 

 it demonstrates other accelerating causes which existed in 

 the case in question, we feel much pleasure in inserting it. 



On examining the clay,^^ it is stated, " at the embankment 

 of the Croydon Railway I find that it contains minute crystals 

 of sulphate of lime fSeleniteJ and these appear to be formed 

 and continually forming by the action of air and moisture on bi- 

 sulphuret of iron, which the clay also very usually contains ; 

 this, by the action just mentioned, is converted into sulphate 

 of iron, with excess of sulphuric acid, and this coming into con- 

 tact with carbonate of lime, commonly existing in clays, appears 



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