Hkndkusox. — Coalfielih of West yehoti. 303 



mauy places ovorlio, perhaps unconformably, the so-called upper s(iams — ■ 

 at any place overlie the lower bituminous seams. All these things point 

 to the conclusion that the coals of west Nelson belong to one series, of what 

 age is here immaterial. Some other agency than time must, then, be looked 

 for to explain the differences in composition of the coals. 



That heat is competent to produce all the changes in coal-composition 

 is well known. Its effects are well shown at Malvern,* where a brown 

 coal has been altered by a volcanic dyke. Such action, however, is purely 

 local, and cannot explain the varieties of coal in west Nelson. Again, 

 the deep burial of coal beneath other rocks, and the consequent increase 

 of temperature, greatly hastens the distillation process. Such a theory 

 is, however, quite inapplicable to west Nelson, where none of the coal- 

 measures are, or appear to have been, deeply buried, and where the occur- 

 rence of the more highly carbonized coals on the higher levels seems rather 

 to contradict the theory. 



Considering, then, crustal movements : these no doubt have great in- 

 fluence both from the pressure exerted and the heat engendered thereby. 

 Probably the anthracite of Fox's River and the plumbago of Pakawauf 

 have been produced by the action of great faults. But if such be the con- 

 trolling factor of this problem it is to be expected that the coal near Nelson, 

 which is actually inverted and entirely crushed by the alpine overthrust. 

 would be highly carbonized. It actually contains 53 per cent, of fixed 

 carbon and 10 per cent, of water. Again, the coals of Blackball and Papa- 

 roa, separated by a fault, which presumably affected them equally, con- 

 tain 50 per cent, and 76 per cent, of fixed carbon respectively. Evidently 

 crustal movements are incompetent to account for the variations in com- 

 position of the coals. 



Time, heat, and pressure have been shown to be inadequate of them- 

 selves. Another condition controlling the rate of distillation is the ready 

 escape of the distillation volatile products. These volatile products would 

 have opportunity to escape if the overlying strata were porous or fissured. 

 Porosity in a rock will have little influence where great thicknesses are 

 concerned. Great thicknesses of porous rock do not, however, overUe 

 the coals of west Nelson, the principal overlying rocks being mudstones, 

 and wherever these have been wholly or in part removed the coals are 

 highly carbonized. 



Applying this hypothesis to the west Nelson coalfields, we find that in 

 the Whakamarama field the coals of the Pakawau section occur in con- 

 glomerates capping the tops of the ranges, the upper more impervious 

 mudstones and limestones, which at one time probably covered them, being- 

 removed. The coals are highly carbonized, but on the western dip of the 

 anticline, and to the north where it plunges, the overlying impervious beds 

 are still in existence, and the coals merge into sub-bituminous and even 

 brown coals. The Taitapu field has always been depressed, and the over- 

 lying impervious covering is being added to, hence the coals are brown 

 coals. Similarly, in the Whakatu field the main central portion of the 

 field will contain brown coal, although round the edges of the basin coals 

 of all qualities may be found, the degree of carbonization depending on 

 local circumstances. In the KaAvatiri field the bulk of the coals will be 

 bituminous, but in parts of the Mangles section and towards the west 

 generally the coals may grade to brown coals beneath the limestones, &c. 



* Evans : Trans. N.Z. Inst., vol. 31, p. 557. 

 t Cox : Geol. Snrv., No. 15, p. 71. 



