Speight. — The Tertiary Beds of the 2' relics-irk Basin. 351 



and early Tertiary times a comparatively thin series of deposits represents 

 an enormous period, and that ample space of time was thereby afforded 

 for great changes in the fauna. 



The main difficulty in accepting the former explanation is that in other 

 localities in fairly close proximity the molluscan fauna of more or less fixed 

 habitat is associated with free-moving forms and with marine reptiles, both 

 of which are likely to have a wide range in space; and if they peopled other 

 parts of the ocean they must, except under most peculiar circumstances, 

 have penetrated the then New Zealand seas, seeing that it is very unlikely 

 that a portion of the sea surface of the world was completely shut off from 

 the general ocean. It is. of course, possible, but improbable. The other 

 explanation therefore appears to me to be more satisfactory. In the area 

 under consideration the thickness of the beds between those containing 

 the Cretaceous fauna and those with a Tertiary fauna certainly amounts 

 to 1,800 ft . — no mean thickness — and the conditions of deposition were in 

 all probability very slow. In discussing the circumstances of deposition 

 it is important to note that the form of the land surface was entirely 

 different from that now existing.* and that the land in the neighbourhood 

 was probably of low relief and did not furnish any large amount of sedi- 

 ment. The greensands and marls were also laid clown in fairly deep 

 water, where deposition would be slow. It seems probable that after 

 the deposition of the coals and estuarine beds at the base of the series the 

 area was depressed, and deposition was slow in the relatively deep water 

 in which the marl was laid down, the unconformity, if any. being due to 

 depression beyond the limit of deposition — that is. a per solium unconformity, 

 and not one due to elevation of the land, erosion, and subsequent depression. 

 That a shallowing of the sea took place towards the close of this period 

 is evident from the presence of the interstiatified sands and conglome- 

 rates, which become increasingly important towards the top of the series. 

 Numerous instances of slow deposition and small thickness of beds in 

 one area associated with great thickness in an adjoining area ran be cited 

 from many parts of the world, but perhaps the most striking one is that 

 afforded by the Silurian beds of Scandinavia, with their compressed 

 though complete sequence, as compared with the beds of the same age in 

 Wales, which are distinguished by their thickness, the former owing their 

 relative thinness to slow deposition in deep water off the shore-line of a 

 continent. 



Volcanic activity became manifest in certain parts of the Trelissick area 

 while the marls and sands were being laid down, especially in the neigh- 

 bourhood of Whitewater and Coleridge Creeks, the eruptions being almost 

 entirely of fragmentary material, and submarine. In close proximity to 

 the centres of volcanic action the limestones thin out and disappear, 

 whereas they are thick in those parts of the area remote from volcanic 

 activity, especially towards the east of the basin. It is probable that 

 while volcanic activity was fairly continuous at one point deposition 

 of limestones was continuous a little distance away, and that in 

 the intermediate localities the limestones and volcanic products are inter- 

 stratified. 



* R. Speight, The Intermontane Basins of Canterbury, Trans. X.Z. Inst., vol. 47, 

 1915, p. 345. 



