333 Geology of 



similar in both localities, the chemical constituents vary so very 

 considerably. Thus, whilst the specimen from near the summit is 

 very rich in lime and almost wanting in alkalis, those from the tunnel 

 contain only the usual percentage of the former mineral, and the 

 amount of soda is even in excess of that usually obtained in these acidic 

 rocks. The other rock, of which mostly all the dykes towards the 

 head of the harbour are composed, is of a domitic nature. It has a 

 whitish claystone matrix, with a number of small crystals of felspar 

 (sanidine). The dykes themselves show a tabular arrangement on 

 both sides, parallel to their direction, the centre being divided first by 

 polyhedric joints, of which the horizontal ones are the most prominent. 

 Each prismatic portion thus formed has generally a spheroidal concre- 

 tionary structure, the central nucleus consisting of a very hard kernel. 

 This latter arrangement is beautifully shown when the spray of the sea 

 has had access to the dykes, through which process decomposition has 

 been much accelerated. A number of gigantic rosettes have been 

 formed, in which yellowish and pinkish tints bring out more vividly the 

 remainiug white colour of the rock, other portions being veined and 

 stained by a dark yellow ferruginous colouration. Fine examples of 

 these rosettes are to be seen in Governor's Bay, near Mr. Potts' resi- 

 dence, and close to Little Eaupaki. Some dykes of a more compact 

 nature consist of a greyish rock containing small crystals of sanidine ; 

 otters have to be referred to the trachydolerites, of which the principal 

 mass is dark ash grey, with numerous crystals of felspar, of which many 

 are striated (oligoclose ?), and in which smaller crystals or grains of 

 hornblende are embedded. The dykes Nos. 44 and 55 passed in the 

 tunnel, and of which an analysis has been made in the Colonial 

 Laboratory, belong to this class. 



There are also some dykes, often of great magnitude, con- 

 sisting of a basaltic rock, of which that remarkable dyke at Sumner, 

 near Morton's Hotel, is a well-known instance. Here the face 

 of the nearly vertical cliff, by which the spur terminates, is nearly 

 200 feet high, and consists of a series of scoriaceous lava-streams 

 and agglomeratic beds, from their nature having offered much 

 less resistance to the waves of the sea, than the dyke passing through 

 them. This dyke has been left standing free for a distance of about 

 100 feet, and is six feet six inches broad, striking from 1ST. 20° E to 

 S. 20° "W-, reaching to the summit of the cliff ; but higher up the spur 

 it cannot any longer be traced, so that in all probability it does not 

 reach to the summit of the caldera wall. Taken along its whole face 



