114 Transactions. 



(9.) Direction, N.W.-S.E. Height above sea-level, 860 ft. Basalt over- 

 lies Signal Hill phonolite horizontally. 



(10.) At Junction School. Heitrht, 920 ft. Basalt lies upon a convex 

 surface of Signal Hill phonolite, dipping both N.W. and S.E. at an increasing 

 angle. 



The diagrammatic section along AA shows the relations of the rocks. 



It is thus seen that the basalt-flow is everywhere later than the phonolite. 

 Moreover, when all the above-mentioned basalt-outcrops are followed up 

 they are found to be continuous with the flow covering the top of Signal 

 Hill, and the flow is continuous across the deep valley to the north. At 

 various levels, also, isolated patches of the same basalt occur on the spur- 

 leading down to Opoho. 



It thus appears that the basalt flowed over a surface quite as uneven 

 as the present surface of the hill, and filled up the depressions. That the 

 basalt did not follow the phonolite in quick succession is proved by the 

 occurrence of a considerable thickness of baked soil on the surface of the 

 phonolite and below the basalt. No direct evidence of erosion, such as 

 the occurrence of phonolite conglomerate beneath the basalt, has been ob- 

 served, but it seems necessary to postulate a long period of erosion before 

 the extrusion of the basalt which then filled up the depressions, and perhaps 

 solidified on some of the slopes without filling up the valleys below. \i, 

 however, the basalt be supposed to have flowed from Mount Cargill, the 

 valley which then occupied the site of the present North-east Valley must 

 have been filled to a depth of 1,000 ft. with basaltic lava. From the lower 

 part of the valley the basalt, if ever it occurred there, has been completely 

 removed by erosion. 



The ridges running down towards the North-east Valley are composed 

 of basalt, while the gullies are eroded in the phonolite, which at the heads 

 of the gullies is at a considerably higher level than the basalt at the lower 

 parts of the ridges. This, together with the fact that the tongues of basalt 

 occur in trough-like depressions in the phonolite, shows that the present 

 ridges occupy the sites of ancient gullies. 



The basalt mass is made up of numerous successive flows. At many 

 points there are interbedded layers of scoria, and also red bands, which 

 appear to be baked surface clays. If this is correct, considerable intervals 

 must have elapsed between the successive extrusions of basaltic lava. These 

 beds or layers cannot, however, be correlated on account of their resem- 

 blance to one another and of the similarity of the basalt of the diiJerent 

 flows. The beds do not appear in any case to lie horizontally, nor to have 

 a uniform dip and strike. 



The strip of basalt filling the upper part of the North-east Valley is 

 surrounded by older rocks at a higher level on all sides except in the direc- 

 tion of the mouth of the valley. Here the older rocks (phonolites) are at 

 a lower level. Hence it appears that the ancient valley into which the 

 basalt flowed had the same general direction as the North-east Valley. The 

 ancient valley was nearly as deep as the present North-east Valley, and, 

 if it be granted that there has been no subsequent tilting, had a gradient 

 not quite so steep. 



A small area of basanite occurring on the Lower North Road scarcely 

 comes into the area discussed. Its relation to the basalt is not apparent. 

 It occurs at a considerably higher level than the neighbouring Signal Hill 

 trachytoid phonolite, and so is considered younger than that rock. It is 

 distant about half a mile from the clear sections on the Main North Road 



