39(5 PROF. P. MARSHALL OX THE SEQUENCE OF [Dec. 1914* 



- IV. Classification of the Rocks. 



Tlie great majority of these rocks fall readily into well-known 

 and generally recognized groups. Thus the lowest rock is a 

 trachyte, obviously an effusive representative of the hostonites. 

 A similar rock from Portobello in this district has been described 

 in the following words by Rosenbusch : 



' Ein reiner Sanidintrachyt ohne femische Gemengtheile mit Einsprenglingen 

 von Sanidin in trachytischer Grandmasse aus Sanidinleistchen.' 1 



In every case in which an analysis of this rock has been made 

 there is so high a percentage of soda, that the felspar can hardly be 

 anything else than anorthoclase, or, at any rate, soda-orthocla.se — a 

 conclusion which is supported by the microscopical examination of 

 my preparations. 



Two of the phonolites (No. 2 & No. 25) belong distinctly to 

 the trachytoid type, and contain very little nepheline. The third, 

 No. 14, has a considerable quantity of nepheline, and is almost 

 nephelinitoid in structure. 



The basalts, too, call for no special remarks, except that they 

 occasionally have some anorthoclase-crystals, which are in all cases 

 much corroded. 



The three lavas, of which No. 13 is classed as a kaiwekite and 

 the other two (No. 21 & No. 24) are classed as trachydolerites, 

 demand special consideration. The name kaiwekite was first 

 used by me in 1906. 3 The characters there stated for that type 

 of rock are the following : — 



(1) The abundance of anorthoclase and of various felspathic intergrowths. 

 (2) The very frequent square and isometric sections of felspar in the 

 ground - mass. (3) The comparative absence of coloured rock - forming 

 minerals. (4) The frequent occurrence of pyroxene, with a brown centre 

 and an aBgirine margin, and of serpentine-pseudomorphs after olivine. 



On the same page it is also stated that the late Prof. Rosen - 

 busch, after kindly examining sections of the rock, informed 

 me (in lift.) that the rock is mineralogically the equivalent of 

 the rhomb-porphyries, and is the volcanic representative of the 

 plutonic laurvikites. The rock here ekssecl as kaiwekite has 

 rather more coloured constituents than the specimens seen by 

 Rosenbusch, but the chemical composition of the rock is so 

 closely similar to that of laurvikite that the suggestion made by 

 him must be adopted. Kaiwekite is included by him among 

 the pantelleritic trachytes. Another slightly more basic type 

 is said to lie between the Ponza and the Drachenfels types of 

 trachyte; 3 



An actual specimen of No. 24, and another rock allied to it, 

 found in the Leith Valley, Dunedin, and previously classed by 

 me as andesite, 4 was by Rosenbusch stated in MS. to be a 



1 ' Mikroskopische Physiogarphie der Mineralien & G^steine : vol. ii, 

 Massige Gesteine ' 4th ed. (1908) pt. 2, p. 927. 



- Q. J. G. S. vol. lxii (1906) p. 400 & pi. xxxix, fig. 2. 

 :t H. Rosenbusch, loc. supra cit. 

 4 Q. J. G. S. vol. lxii (1906) p. 408. 



