Marshall and Murdoch. — Tertiary Rocks near Wanganui. 119 



thickness between Kai Iwi and Nukumaru is 1,050 ft., and that between 

 Nukumaru and Waipipi is 1,450 ft. If the rate at which these sediments 

 were deposited can be approximately estimated, it should be possible to 

 calculate the time at which various species of mollusca made their last 

 appearance. It is generally estimated that the average rate of deposition of 

 sediment amounts to about 1 ft. in 100 years (Sollas, 1905, p. 24). This 

 estimate, of course, cannot have a precise application in all actual cases, if 

 in any one, because in every case the true rate must depend upon the size 

 of the country that is being denuded and is supplying the sediment, upon 

 the hardness of the rock of which this country is mainly composed, and, 

 of course, upon the distance from the coast-line of the area in which deposi- 

 tion is taking place. 



In the present instance the area of land which was undergoing denu- 

 dation was probably small, and therefore supplied sediment at a slow rate. 

 Whether the land was composed of granite rock or was in part formed of 

 sediments that had been derived from granite and raised above the sea- 

 level a little while earlier and again submitted to denudation is by no 

 means certain. On the one hand, the fact that from Karioi to Waiouru, 

 sixty miles inland, rocks of this very young Tertiary age were being 

 deposited points to the conclusion that there was at this time a wide extent 

 of shallow sea in the Wanganui district. On the other hand, the inter- 

 bedded land-surface near the mouth of the Okehu Stream supports the 

 idea that a part at least of the huge Tertiary area to the north was above 

 the sea-level for a portion at least of the period of deposition. If that was 

 the case sediment would have been supplied at a relatively rapid rate. 

 Until the country to the north has been more fully examined with the object 

 of ascertaining how large an area is bare of a covering of these . youngest 

 Tertiary rocks it is not safe to offer any opinion on this matter. Much of 

 the sediment was deposited at a considerable distance from the coast-line, 

 and in water of considerable depth. This is proved both by the fine- 

 grained texture of the sediment and by the nature of the fossil mollusca 

 embedded in it. The sediment ' is generally of an extremely fine grain 

 and contains a large proportion of smaU mica plates, two features which 

 point to deposition in an area which was reached by the finest and 

 lightest sediment only. 



The fossil mollusca found over the first five miles north of CastleclifE 

 are of a nature which indicates that the sea-floor was at a depth of between 

 50 and 100 fathoms. At Kai Iwi the depth of the water was probably 

 nearer the former than the latter figure. If this were the case the rate 

 of deposition must have been a very slow one. Thus the following 

 considerations point to the probability of slow deposition : (1) The small 

 size of the land area from which the sediment was derived ; (2) the hard 

 nature of the rocks that were undergoing denudation ; (3) the distance 

 of the area of deposition from the area, of denudation ; (4) the depth of 

 water in which the deposition took place. On the other hand, it is possible 

 that-^(l) some of the material was derived from older Tertiary rocks only 

 lately deposited (if that were the case the deposition area may have been 

 quite close to that of denudation) ; (2) some of the strata were deposited 

 under shallow-water conditions and close to the shore-line. A balancing 

 of these considerations inclines us to the opinion that the material was 

 possibly deposited at the rate of 1 ft. of sediment in 200 years. If this rate 

 of accumulation is applied generally to the thicknesses of sediment that 

 have been already mentioned, we find that the Waipipi beds are 690,000 

 years older, those at Nukumaru 400,000 years older, and those at Kai Iwi 

 190,000 years older than the CastleclifE beds. 



