336 Transaction*. 



are less common, and only Cristellaria could be distinguished. There is 

 some LithothawnivM."* Hutton would discriminate between the two beds 

 by insisting on the coralline nature of the upper as distinct from the non- 

 coralline nature of the lower, but a careful examination shows that the 

 coralline element may occur in both ; in fact, Hutton himself has noted 

 the coralline nature of the lower limestone at the upper Porter limestone 

 gorge. Even where, through pressure as a result of intense folding or 

 faulting, the rock has taken on a crystalline facies traces of its coral- 

 line origin are apparent. This can be well seen at Parapet Rock and in 

 Coleridge Creek. The lower portions apparently contain a considerable 

 mixture of volcanic-tuff fragments, and are more distinctly bedded in 

 places ; especially is this the case in the limestones exposed in the gorge 

 of the Porter. In the upper portions the planes of bedding are less pro- 

 nounced and the coralline fragments are more distinct, so that a decidedly 

 different appearance is given to the rock. This led Hutton and McKay 

 to class them as two distinct stones, and the absence of the decidedly coral- 

 line facies from certain localities led them to attribute its non-existence 

 to erosion. They therefore placed an unconformity between the higher 

 limestone bed and the overlying calcareous sands and shell beds, and the 

 divergence in character between the upper and lower beds led them to 

 place an unconformity between them, thinking also that the volcanic tuff 

 marked an erosion surface. In my opinion they are members of one lime- 

 stone, for the following reasons : (1.) There is no stratigraphical break ; 

 wherever they occur the beds are conformable, and when there is no tuff 

 it is impossible to recognize a distinct break, but one bed passes into another 

 insensibly. (2.) The characters of the two rocks are not invariable. As 

 Hutton pointed out, the lower bed is in places distinctly coralline, and the 

 upper bed, even when there can be no possible doubt as to its identity, 

 may show an entire absence of this feature. (3.) The palaeontological 

 evidence will show that there is no great divergence in fossil-content between 

 the tuff bed under the lower limestone, classified as Cretaceous by Hutton, 

 and the tuff above it, which he assigned to the Oligocene period, the con- 

 formity of the upper limestone to the upper tuff being generally admitted. 



For these reasons I consider that there is only one limestone ; that it 

 is thinnest where volcanic action was most marked- — that is, in Coleridge 

 Creek ; that in the deep water removed from the volcano sedimentation 

 went on continuously, with a gradual change in the nature and condition 

 of the organisms contributing to the formation of the rock, but that in 

 closer proximity to the volcano interstratification of the limestones and 

 the tuffs took place. .Whereas in the lower horizons the limestone had 

 more or less the character of a deep-sea deposit, as time progressed it was 

 laid down in shallower water in preparation for the next suite of calcareous 

 beds, which have the nature of shore material. 



(c.) Pareora Beds. 



The next succeeding suite of beds, called " Pareora " by both McKay 

 and Hutton, is considered by them to rest unconformably on the under- 

 lying series, a conclusion based chiefly on the supposition that the upper 

 bed of limestone has been removed by erosion from certain parts of the 

 area. If, however, there is only one limestone, then this argument falls 



* P. Marshall, The Younger Limestones of New Zealand, Trans. N.Z. Inst., 

 vol. 48, 1916, p. 92. 



