544 F. Hardy — Mineral Composition of Fenland Silt. 



that the behaviour of the mineral in those soil processes involving 

 the chemical action of a base, might be somewhat different from that 

 of, say, calcium carbonate. 



Hitherto chemical analyses of soils have included a determination 

 of the total carbonate content (expressed as CaCOs) irrespective 

 of the mode of occurrence of carbonate The usual method employed 

 for carbonate estimation in soil-analysis is that described by Amos.^ 

 In this method the soil carbonates are decomposed by acting on a 

 known mass of the soil with fairly concentrated hydrochloric acid, 

 which is slowly heated to boihng point. The carbon dioxide evolved 

 is absorbed in caustic soda solution contained in a Reiset tower, 

 and its amount estimated by double titration with standard acid, 

 using first phenolphthalein and then methyl orange as indicator. 



The value of the method lies in the fact that it gives accurate 

 results with soils which are suspected of being on the border-line of 

 deficiency of calcium carbonate, and hence it furnishes the soil- 

 chemist with data upon which to instruct farmers as to the 

 advisability or otherwise of liming arable land. 



The prolonged boiling with hydrochloric acid involved in the 

 estimation is sufficient to decompose any carbonate which is likely 

 to be present in soils. It does not differentiate, however, between 

 easily decomposed carbonates, such as the various forms of calcium 

 carbonate (calcite, aragonite, etc.) and the more stable carbonates, 

 such as magnesite, dolomite, and minerals allied to the latter, 

 slderite, FeCO^, ankerite, CaCO, . (Mg, Fe)CO,,; mesitite. 2MgCOs '. 

 FeCOs, breunnerite, (Mg, Fe)CO,. etc. 



The inquiry naturally led to two distinct lines of investigation, 

 one of a geological and mineralogical nature, and the other of 

 chemical and agricultural bearing. 



In the present paper it is intended to deal with the geological 

 aspect of the subject. The results of the purely chemical side of the 

 work, with its attendant agricultural applications, are set out in 

 another place. ^ 



In order to account for certain ])eculiarities of the Fenland silt 

 soils, it would perhaps be advisable first to summarize what is known 

 of the origin and method of formation of the Fenland silts. The 

 geology of the silt lands is described by S. B. J, Skertchley ^ in a 

 Memoir of the Geological Survey, from which the following account 

 is taken. 



II. GEOLOGICAL HISTORY OF THE FENLAND. 



The geological history of the Fenland commences with the 

 breaching of the one-time continuous escarpment of Chalk, which 



1 A. Amo.s, Journ. Agric. Sci., vol. i, 1905, p. 322. 



2 F. Hardj% " A Preliminary Investigation into the Occurrence of difl'erent 

 kinds of Carbonates in Soils," to be published in Journ. Agric. Sci., vol. xi, 

 parti, 1920. 



3 S. B. J.Skertchlej^ " The Geology of the Fenland" (Mem, Geol Surv.), 1877. 



