432 Transactions. 



Note on Pseudamusium huttoni. 



This is the most abundant and characteristic of the molluscs 

 of the Dunedin Tertiary sandstones. It was originally identified 

 by Hutton as Pecten hochstetleri described by Zittel (" Reise 

 der ' Novara ' — Palaeontology," tab. xi, figs. 5a and bb), but, as 

 Professor Park* has lately pointed out, the shell does not cor- 

 respond with Zittel's Pecten kochstetteri, the latter having one 

 valve ribbed, while the former has both valves smooth. He 

 has therefore given it its present name. A sketch of this shell 

 is shown in Plate IX, fig. 1. 



Petrography. 



The volcanic rocks of Harbour Cone, like almost all those 

 of the volcanic portion of the Otago Peninsula, show by their 

 constituents a clear derivation from an alkaline magma, with 

 the exception of the dolerite occuring at the summit of the 

 mountain, of which the minerals do not indicate any alkaline 

 chemical composition. The characteristic minerals of all are 

 in many cases very strongly alkaline, the persistent occurrence 

 of aegerine being most striking. The rocks, though differing 

 in their mineralogical characters, indicate by their mineral 

 composition a very close relationship in their chemical com- 

 position. These general resemblances point to the fact that 

 the rocks come undoubtedly from one common magma situated 

 below the district, and any differences in mineral composition 

 must be explained by one of the theories of the differentiation 

 of magmas. It is more than likely that a differentiation or a 

 very great local variation in magmatic composition will account 

 for the presence of the dolerite in such close proximity to this 

 collection of alkaline rocks, as the difference in mineral com- 

 position is no greater than has been recorded in other flows not 

 only in the same neighbourhood but issuing from the same vent. 



For determining the plagioclase feldspars the following 

 method was employed : Crystals showing albite twinning were 

 selected cut as nearly at right angles to the composition plane 

 as possible, which was indicated by adjacent lamellae extinguish- 

 ing between crossed nicols at right angles on either side of the 

 albite plane. Several crystals were measured, and the maximum 

 value for the extinction was compared with the tible given by 

 M. Levy (" Etude sur la Determination des Felspaths dans les 

 Plagnes minces, 1894," pt. ii, p. 29, et seq.). 



Many of the rocks are nephelinitoid, but the nepheline is 

 often ultra-microscopical. In cases where its presence was 

 suspected a portion of the very finely ground rock was treated 



* Paper read before the Otago Institute, 9th September, 1904. 



