Wild. — A Soil Survey of New Zealand. 



481 



series. Lastly, we may refer to Foreman's paper,* which shows that the 

 Oxford clays of Cambridgeshire are phenomenally rich in CaC0 3 . 



" Again," says Rigg, " mechanical analysis at once reveals the neces- 

 sity for a subdivision into two soil formations, since there is a constant 

 difference of 9 per cent, in the percentage of clay found in the two soil 

 formations mapped from field observations." This is true, .and it shows 

 that this clay loam formation, though derived from the Oxford clay, is, as 

 judged by the mechanical analyses given in the appendix, really quite like 

 the clay loam formation of the boulder clay series and the glacial clay ; 

 while the pure Oxford clay is quite similar to the pure Gault clay. 



The table showing the distribution of crops (abridged below) brings to 

 light the same fact : the clay soils, of whatever geological formation, mainly 

 carry legumes and cereals, while the clay loams, of whatever geological 

 formations, are characterized by a greater variety of crops. 



Distribution of Principal Crops, Market-garden District of Biggleswade. 

 (After T. Rigg. Figures give percentage of total area occupied by crop.) 



Crop. 



Pure-clay Soils. 



Oxford. Gault. Boulder. 



Clay Loams. 



Oxford. 



Boulder. 



Brick- 

 earths. 



Glacial. 



Rigg's account of the " characteristic properties and agriculture " of 

 the soils emphasizes the same facts — that agriculturally clay soils, of what- 

 ever geological origin, are alike ; and the same applies to loams. 



Referring to the Gault clay, Rigg says that the " content of calcium 

 carbonate distinguishes it from the Oxford clay formation." To prove 

 this, however, he cites only one analysis of each. It may also be noted 

 that two soils of the three analysed from the " redlands " formation (Gault 

 clay series) have only 0-05 per cent, and 0-09 per cent, of CaC0 3 . More- 

 over, Hall and Russell's analyses show that the Gault clay soils of their: 

 district are by no means rich in CaC0 3 — one has as little as 0-01 per cent, 

 —so that Rigg's generalization from the analysis seems totally unjustified. 

 Finally, Rigg concedes that in other respects the Oxford clay " resembles 

 the Gault clay very closely." 



After a careful review of Rigg's paper the present writer concludes 

 that in classifying for agricultural purposes the soils therein described — ■ 

 (1.) Geological origin is relatively unimportant. 

 (2.) Mechanical analysis and physical structure are all-important. 

 (3.) The clays of all three geological formations resemble one another 

 in more numerous and in more important points than they differ 

 from one another ; and the same statement applies to the loams 

 and to the lighter soils. 

 (4.) A simple classification showing three series only — clays, clay loams, 

 sandy loams — would be more useful from the agricultural stand- 

 point. Subdivision of these series into "facies," determined by 

 geological origin, might follow if desired for detailed description. 



* F. W. Foreman, Soils of Cambridgeshire, Journ. Ag. Sci., vol, 2, 1907, p. 161. 

 16— Trans. 



