412 PEOP. P. MARSHALL ON THE [Aug. 1906, 



the groundmass very fine. At Papanui Inlet, and at a few other 

 localities, hornblende-basalts occur, showing the ordinary dark- 

 brown hornblendes with a resorption-border. 



Occasionally, types are found in which a small quantity of brown 

 glass remains ; and at Sandy Mount there are fragments in a scoria- 

 bed that consist almost solely of a brown glass, in which are 

 dispersed some felspar-microliths and magnetite-grains. 



In the upper part of the Leith Valley occurs a very basic type. 

 It contains large inclusions of olivine, which in section show some 

 picotite and diopside. Smaller olivines are numerous. There is no 

 felspar ; but granular augite and magnetite constitute the remainder 

 of the rock. 



In nearly all instances, the olivine of the basalts is more or less 

 decomposed. Usually the change has resulted in a mere staining 

 with limonite ; but in many cases there is a more or less complete 

 change to serpentine. 



There is no striking chemical peculiarity in the basalts, so far as 

 they have been examined, except that the percentage of alumina is 

 rather high. A quantitative analysis of an ordinary example has 

 been made, with the following result : — 



Si0 2 45-28 



A1,0 3 1901 



Fe'oOg 8-52 



FeO 501 



MgO 5-14 



CaO 7-55 



Na a O 266 



K o 251 



U' 2 299 



Total 98-67 



Very many lava-flows and some dykes of basalt occur in the 

 district. No attempt has been made to separate the types of basalts 

 into groups, although it appears as if the nature of the dominant 

 pyroxene might be taken as a basis. 



There is no doubt that some of the basalts are intrusive, and 

 represent pipes of former small cones. This is well seen at St. Clair, 

 Black Head, and at Mount Cargill, where the northern peak is 

 formed of a columnar basalt, while in the first two localities the 

 basalt-neck penetrates through the calcareous sandstone. 



From the section at North Otago Head, it is evident that basalt- 

 flows followed the effusion of trachyte. They were succeeded by a 

 flow of trachytoid phonolite. Basalt again succeeded, with a brief 

 change to basanite, and finally a second trachytoid phonolite. 



Basalts appear to have been almost the last of the lavas emitted, 

 for basalt-flows cover the conglomerates at Kaikorai. Waikouaiti, 

 and at Union-Street Bridge ; and pebbles of trachytoid phonolite, 

 and of nearly all the other types of rock, occur abundantly in the 

 conglomerates. The andesitic type of trachytoid phonolite is poorly 



