STRATIGRAPHY. 33 



of lamination, break off with a vertical face, and sometimes are 

 penetrated by numerous small, sinuous, and branching tubeletsr 

 When this structure is present they resemble the typical loess of 

 the Rhine, but more often they form an amorphous loam. 



The laboratory of the Geological Survey is not equipped with 

 apparatus for the physical analysis of soils, but 



Mechanical composition. . . . 



an approximate determination of the nature of 

 this deposit was made by subjecting some samples to sieving and 

 passage through an elutriata, which was found by experiment to 

 remove sand grains of less than *oi mm. (^--J-^ inch) in diameter. 

 On passing the samples through this it was found that from 70 to 90 

 per cent, was washed away and that the greater part consisted of 

 ferruginous and calcareous concretions, and only about '5 to 2 per 

 cent, of mineral grains. Of the 70 to go per cent, which was removed 

 by the elutriata a large proportion settled in still water if left standing 

 for a couple of minutes, but a marked turbidity of the water remained 

 and a notable proportion was fine-grained mud which settled, in water, 

 at a rate of not more than an inch in a minute. 



It is remarkable that even this fine-grained mud consisted in 

 part of grains of carbonate of lime, and effervesced on treatment 

 with mineral acid. 



It is impossible to account satisfactorily for the formation of these 

 deposits by water. The material of which they 



Not of aqueous origin. ■ * 



are composed would only settle in water that 

 was absolutely still, but they present none of the characteristics of 

 a fine-grained sub-aqueous deposit. Into a lake or pool mud is 

 washed down at intervals by floods, the coarser grains settle first, 

 followed by the finer grains which subside more slowly, thus produc- 

 ing a laminated structure which is conspicuously absent in the recent 

 deposits under consideration. In a river valley and as the result of 

 alluvial deposition such fine-grained,, unlaminated deposits are im- 

 possible, except as very local accumulations. 



The surface contour is equally inconsistent with their aqueous 



origin. Instead of forming a level plain, sloping 

 Surface forms. , S „ , ,. , . „ 



down the valley, but extending horizontally on. 



D ( 33 ) 



