36< • Transactions. 



able explanation is that it is due to the alteration of organic matter. This 

 exists in two forms at the base of the Cretaceo-tertiary series — viz., as 

 coals and lignites of vegetable origin, or as animal matter of marine origin 

 Although the former is a possible source of the supply, it appears extremely 

 unlikely that these coals have been depressed so far that the earth's internal 

 heat has been sufficient to convert them into hydrocarbons. Our coals 

 show little change unless they have been placed in close proximity to volcanic 

 intrusives, in which case they are converted into altered brown coals or into 

 mineral cokes or anthracites. The marine-organism explanation, therefore, 

 is apparently the best solution of the problem. Below the limestones of 

 Lower Tertiary or Upper Cretaceous age there lie almost invariably Green- 

 sands and other sands which contain a considerable amount of matter of 

 organic origin. It has been noted previously that the black oysters asso- 

 ciated with the coals of this series, when broken or struck with the hammer, 

 give off a pronounced petrolaceous smell, and the remains of Foraminifera 

 and other marine organisms are widely distributed through the Greensands. 

 A thick band of these contains very generally a large quantity of sulphur, 

 whose formation can certainly be attributed to the former presence of 

 organic matter; and the frequent association of petroleum with sulphur 

 compounds in other fields is also very suggestive in this connection.* A 

 very illuminating paper by Daly in the " American Journal of Science," 

 vol. xxiii, 1907, entitled "The Limeless Ocean of Pre-Cambriau Time," 

 draws attention to the probable accumulation of organic matter which may 

 occur under conditions similar to those obtaining in the Black Sea at the 

 present time, and to the existence of large quantities of sulphur compounds 

 associated with these accumulations. We have thus an explanation of the 

 connection of sulphur with the large masses of organic material which could 

 yield petroleum if buried under an impervious covering and subjected to the 

 slow but sure chemical changes which go on in the crust of the earth, whether 

 these changes be due to the transference of heat from lower levels in the crust, 

 or to the action of bacteria, or to any other cause. 



If, then, these sands and their associated beds be the locus from which 

 the supplies of petroleum are derived, it becomes increasingly important 

 when prospecting for oil in this country to know what thickness of the Mio- 

 cene marls has to be penetrated in order to reach the underlying Cretaceous 

 sands. In the North Island the marls are very thick — up to 5,000 ft., and 

 perhaps much more. In the South Island the marls, with the exception 

 of those in the Marlborough district, are comparatively thin ; but they are 

 usually folded with broken anticlinal crests, and it is probable that the 

 greater part of the oil they may have once contained has escaped. There 

 are places, however, such as Cheviot, which do undoubtedly give indications 

 of the. presence of oil in small quantities, and in these cases it may be reason- 

 ably supposed that the conditions favoured its retention for a longer period 

 than elsewhere. 



* See U.S. Geol. Surv., Bull. 398 : " Geology and Oil Resources of Coolinga District, 

 California," p. 187. 



