478 Transactions. 



of organic matter and given the underlying differences full play. The 

 character of our climatic zone is such as to accentuate variety of soils, the 

 humidity is considerable and carries the degradation of the rock material 

 so far that there is an enormous range in the sizes of the particles making 

 up the soil, from coarse sand grains down to clay particles of the order 

 of colloids ; there has also been sufficient accumulation of humus to modify 

 the texture of the mass and make it work as a unit and nat as a 

 loose aggregate of powdered rock. Chemically also our soils lie between 

 the semi-arid soils, with their richness in unleached salts derived from the 

 decayed rock, and the ' podzols ' from which all soluble material has been 

 washed away." 



The first method, the geologico-petrographical classification, and the 

 second method, the chemico-petrographical classification, may conveniently 

 be considered together, since they have usually been employed in con- 

 junction with one another in Great Britain, where the adoption of the 

 combination by Hall and Eussell in their Agriculture and Soils oj Kent, 

 Surrey, and Sussex* has firmly established it. The authors " have assumed 

 that each [geological] formation represented in the district will give rise 

 to a soil type which can be characterized both by its mechanical analysis 

 and by special features in the farming which prevails over its outcrop. 

 The justification for these cardinal assumptions was obtained in the early 

 stages of the work by following the dividing-line representing the outcrop 

 of two formations, and finding (1) that the dividing-line held for the 

 soils as well as for the underlying formations ; (2) that the soils from 

 any formation (with one or two exceptions) did show on analysis certain 

 common features which marked them off from the other soils. These con- 

 clusions have been strengthened as our work proceeded ; all our experience 

 in the field goes to show that each formation in the area under consider- 

 ation gives rise to a distinct soil type, the characteristic composition of which 

 can further be recognized by making up an average from the mechanical 

 analyses of the samples taken from the formation. Even in such a case 

 as that afforded by the Lower Wealden soils, which vary from something 

 near a sand to a heavy clay, there still exists but one type of soil, possessing 

 very marked and special characters, though subject to a considerable range 

 of variation from light to heavy." 



Since the publication of this work surveys of various comities have 

 been completed or taken in hand in Great Britain. Practically all of this 

 work, especially that done in England, follows consistently the methods 

 of Hall and Bussell. Thus in that most recently published — namely, a 

 paper by Riggt — -we read, " The writer has followed Hall and Russell in 

 using the geological formation to mark the extent of a series of soils which 

 have a somewhat similar mineral structure. These series of soils have, 

 however, then been separated into soil formations having different agri- 

 cultural properties, and the extent of each has been mapped." 



As one of the objects of the present paper is to show that in general, 

 and more particularly in New Zealand, the geological map is not the 

 best basis of a soil survey (in other words, that the geologico-chemico- 

 petrographical method is not the best method of soil-classification), a 

 detailed consideration of some of the British work is necessary. 



* Published by the Board of Agriculture and Fisheries, 1911. 



t T. Rigg, The Soils and Crops of the Market-garden District of Biggleswade, Journ. 

 Ag. Sci., vol. 7, 1916, p. 385. 



