Speight. — Occurrence of Petroleum in New Zealand. 357 



verv strong, so that any explanation of its origin must account for its occur- 

 rence near New Plymouth as well as in parts distant from that town, and 

 must also be considered in connection with its distribution in beds of similar 

 age and lithological character elsewhere in New Zealand where the con- 

 ditions obtaining at Moturoa do not exist. 



The first explanation of its origin was that adva.net d by Sir James Hector, 

 who suggested that the petroleum was formed by the distilling action of the 

 heated rocks of Paritutu and the Sugarloaves on the brown coal, which, 

 it was supposed, continued from the Mokau River till it came in close 

 contact with the igneous masses near Moturoa. 



This hypothesis has been accepted by most geologists who have written 

 on New Zealand geology, including Park and Bell. McKay Las. however, 

 considerable doubt about it ; and Clarke, too, hesitates to accept it. but does 

 not advance any theory of his own. That the volcanic rocks have had some 

 effect is no doubt true, judging from the remarkable amount of carbon 

 dioxide present in the gas given off in the wells (see analyses quoted, p. 46, 

 Bull. No. 14, N.Z. Geol: Surv.), these results differing markedly from those 

 given for other wells in the Taranaki District. The chemical constitution 

 of the oil itself shows a notable percentage of constituents distilling at a 

 nigh temperature, and points either to the more volatile matter having 

 Deen driven off by the heat of the volcanic masses or its having escaped 

 through fractures in the rocks Both of these circumstances may occur 

 in the same locality, and in this case may be attributed to the igneous masses 

 of Paritutu and the Sugarloaves, which have provided the requisite heat, 

 and have also in all probability fractured and dislocated the surrounding 

 beds when the intrusion took place. 



In criticizing Hector's theory it may be pointed out, — 



(1.) There is no evidence that the Mokau brown-coal measures do 

 extend from the outcrops on that river to the neighbourhood of New Ply- 

 mouth — a distance of approximately seventy miles — and, judging from the 

 form of the coal areas in other parts of New Zealand, this is extremely 

 unlikely to be the case. Most of these areas in New Zealand are marginal 

 in character, and rarely continue over any wide expanse, although their 

 length may be considerable : the seams, too, are lenticular in shape and are 

 markedlv discontinuous. If, however, there were other concealed coal 

 areas lying on a pre-Miocene land-surface in the neighbourhood of north- 

 west Taranaki. this objection would not apply. 



(2.) Seeing that the action of volcanic heat cannot be responsible for the 

 occurrence in districts remote from the centres of volcanic activity, some 

 other cause must be looked for in order to account for its presence ; and 

 because the petroliferous beds near Moturoa are of the same age as those 

 containing oil in other parts of the country, it seems reasonable that they 

 owe it primarily to the same cause, even if local variations of conditions 

 exercise a modifying effect. In advancing any theory for the origin, there 

 is an important question to be considered — viz.,- whether the oil has ori- 

 ginated in the papa beds or has migrated into them from some, other beds, 

 necessarily of more deep-seated position. It is likely, too, that both of 

 these circumstances may be in existence at once, and that the beds owe part 

 of the amount to accumulation in situ and part to migrations from other 

 strata. 



The fact that the Miocene mudstones are usually associated with petro- 

 leum indications, and also that where these beds do not occur the indications 

 are scanty or absent, leads one to regard them as the locus of origin. As 



