Marshall, Speight, Cotton. — YouiKjer Jioch-series of N .Z. 401 



gravels in W'estlaiid. It is also true that the greater part of the sei'ies was 

 deposited on a slowly subsiding sea-floor, and was derived from a slowly 

 subsiding land on which all the stream-grades were thus being gradually 

 I'educed. and most of tlu^ material was doubtless utilized in filling up pre- 

 viously eroded valleys. At the same time the area from which sediment 

 could be derived was constantly being reduced. A more detailed descrip- 

 tion of the different strata is, however, necessary in order that the slow 

 rate of deposition should be fully realized. 



The basal gravels are, as previously stated, 7.000 ft. thick in Westland. 

 They are certainly more than 1,000 ft. thick near Cape Farewell, and perhaps 

 3,000 ft. thick at Shag Point. This thickness points to a considerable lapse 

 of time In most localities where they are relatively thin the component 

 pebbles show evidence of great and prolonged attrition. This is most 

 marked in Otago, where the gravels are composed almost wholly of quartz 

 pebbles, and they are unquestionably derived from rocks of mica-schist. 

 Th* rivers that flow from the schist at the present time carry down schist 

 pebbles amongst which are a few of quartz. A great amomit of attrition 

 of the schist and a gradual survival of the hard quartz pebbles would be 

 necessary before the change from the schist pebbles to a quartz gravel 

 would result. This is equivalent to the statement that the rivers supplied 

 material more slowh^, and that it was subject to most prolonged attrition 

 on the beaches before it was screened by a new supply. The lowest stratum, 

 therefore, by its nature or by its thickness indicates that much time elapsed 

 during its deposition. 



(ii.) The Coals. — Interstratified with these conglomerates there are in 

 many places beds of coal which are sometimes of great thickness. Those 

 certainly indicate a slow rate of deposition, and imply long periods of nearly 

 stationary conditions separating the periods of more rapid depression. 



As the depression proceeded the area of land must have been greatly 

 decreased, and this satisfactorily explains w^hy in some localities an aren- 

 aceous limestone rests directly on the eroded surface of the older rocks. 

 This is the case at Kawhia, and on the Gouland Dowiis iu the west of the 

 Nelson Province. The search for geological indication of the presence of 

 coal-seams has perhaps more than any other reason been the assignable 

 cause for the close examination to which the New Zealand Cainozoic rocks 

 have been submitted. In all places where coal has been foimd in these 

 rocks it has been near the base of the series, interstratified with the con- 

 glomerates. It is, however, of local extent, of very variable thickness, 

 of variable composition, and occurs at very different levels. The fossil 

 MoUusca associated with the coal in different places are, however, dis- 

 tinctly different. This has been the reason for assigning the seams to very 

 different ages. Hector classed some in the Cretaceous, but the greater 

 part in the Cretaceo-tertiary at the horizon of the black grit. Hutton 

 considered several coals of Cretaceous age, others Oligocene, and some 

 Miocene, in each case forming the base of the younger series of rocks in 

 the districts where they occur. 



Not only have ihey been classed in various ages by different observers, 

 but the same observer has placed them in very different ages in different 

 publications, and the whole subject of the coal-seams has thus become of a 

 complex and confusing nature. To illustrate this it is only necessary to 

 refer to the following works of Park in reference to New Zealand coals : — 



" The Extent and Duration of Workable Coal in New Zealand " (Park, 

 Trans. N.Z. Inst., vol. 21, p. 327, 1889) : " The workable coals of New 



