244 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 30, 



teriform shape. The trachyte-rocks stand in a huge mass on the 

 flank of the Tertiary formation, rising high above them with fan- 

 tastic, in some cases overhanging peaks. 



The surfaces of the boulder-rock and the trachyte are of very 

 compact texture, seeming to indicate the existence, as the mass 

 cooled, of immense pressure as well on the sides as on the summit, 

 occasioned by matter perhaps of a more destructible nature which 

 has since been removed, probably by denudation. 



II. Respecting the second class of eruptions, the lavas of which 

 constitute part of the Tertiary series, the general characteristics are, 

 first, a great smoothness or worn- down appearance of the cones and 

 craters; the cup having been filled up, and the brim having been 

 broken away. The points of eruption are indicated usually by some 

 slight hollow, but chiefly by the streams of basalt and scoriffi that 

 centre there. The whole cone, consisting probably of loose cinders, 

 has been washed away, and its remains are spread along for miles, 

 in some cases, between the beds of clay ; carrying with it fragments 

 of Tertiary rock, unaltered, but rounded : beds of indurated mud are 

 again superimposed on these. The volcanos Nos. 24 & 51 on the 

 Geological Sketch-map (PI. XII.) are instances of this. 



Of these Tertiary volcanos some have, perhaps, been not altogether 

 subaqueous, but have raised their cones above the water, as in the 

 case of the North Head at Auckland Harbour (No. 5 on the map, 

 PL XII. See also the drawing, PL XIII.). 



In this case no clays are superimposed, but the surface over the 

 lower beds of ashes is of that horizontal character which indicates the 

 action of water as the ashes fell, or before they were consolidated. 

 Around the sides of this crater, the tails of the volcanic bombs are 

 more perfect (less injured by the fall) than could have been the case, 

 I think, if they had descended into anything but water. The lavas 

 of the submarine eruptions appear more compact than those of the 

 recent volcanos. Nothing like cellular scoriae has yet been found 

 among the cinders of this class. 



III. The third class of volcanos here may be considered to be those 

 that came into eruption when the Tertiary was upraised. They lie 

 on the edge of cliffs, or on the prolongation of the line of a cliff that 

 has dipped into the sea, as in the sketch (fig. 1). 



The lavas of these have an older and more decomposed appearance 

 than those of the fourth class, and the craters have always broken 

 out towards the lower or seaward side. 



In one instance, where there is a remarkable fault in the Tertiary 

 rock, eruptions and a crater have resulted, the deranged strata dip- 

 ping towards the point of eruption (fig. 2). 



The fourth class, or those eruptions that have come up through 

 the already upheaved Tertiary rocks, show the greatest variety of 

 form and conditions, — a result perhaps only attributable to their 

 having been less affected by time and disturbances. 



IV. The volcanos of the fourth class may again be systematized 

 as follows : — 



1. Tufa-craters, of but very slight elevation. 



