1859.] HEAl'UY VOLCANIC COUNTRY, NEW ZEALAND. 243 



ceous formation, and lastly, a magnetic sandstone-rock, mixed with 

 a black conglomerate. This series rises into hills of 800 or 1000 

 feet above the sea. 



To the northward of the isthmns the Tertiary is bounded on the 

 eastern slope by a black trap-rock of a very close texture, next by a 

 black boulder-rock, and finally, on the west coast, by a trachytic 

 breccia, rising into peaks and ridges of from 700 to 1500 feet high. 



To the eastward of the isthmus are several islands, in the Gulf of 

 the Thames, composed of clay-slate, of basaltic lava, and of the black 

 boulder-rock. The latter rises into peculiar sharp crags, at a height 

 of 1000 feet or thereabouts. 



The isthmus may be considered as a basin of Tertiary rock. 

 Through it have burst up, dotting its surface, as many as sixty-two 

 separate volcanos ; showing in nearly every instance a well-defined 

 point of eruption — generally a cup-like crater, on a bill about 300 

 feet high above the plain. 



In some instances there are as many as four points of eruption in 

 the compass of a square mile, — the streams of lava commingling or 

 overlapping ; and the former crater in some cases filled up by the 

 ashes from the more recent one. 



On an examination of these volcanos, differences of age become 

 at once apparent ; and the relative position of their respective beds 

 of ashes in the surrounding rocks facilitates the inquiry as to their 

 priority of eruption. They may be classed as follows : — 



1st. The eruptions, on a stupendous scale, of the mountain-masses 

 with boulder-rock, rising to a height of 1000 or 1500 feet above the 

 Tertiary basin ; and perhaps coeval with this was the rising of the 

 trachytic breccia. The relative ages of the black boulder-rock and 

 the trachyte, in respect to the Tertiary beds, must remain for a time 

 doubtful. At present there is no appearance of the trachyte having 

 been more recent than the Tertiary, save that it is in one place 

 superimposed ; and this, perhaps, is only its debris, consolidated. The 

 trachyte shows no difference of texture below or above. There are 

 abundance of dykes in it, but no craters; and while it lias risen to 

 a height of 1 W)0 feel in peaks, there is no high mountain on any 

 siile to wall-iu the igneous mass. The peaks have not in any way 



the appearance of the broken parts of the brim of a crater; they 



rather look like the hardest parts of dykes, the softer contiguous 



r "1 having disappeared. 



2nd. Subaqueous eruptions through the Tertiary beds at the time 

 when they were yel submerged. The allies of these eruptions form 

 horizontal and extended beds below some of the Tertiary clays, and 

 are conspicuous for miles along the cliffs on the easi of the basin. 



3rd. Eruptions that have occurred at the upheaval of the Tertiary 

 beds. These are generally situated on the line of the cliffe, or over 



faults in the Tertiary .strata : and, 



Ith. Eruptions through the Tertiary strata. 



I . ( »f the first class ( the black boulders ami trachytes i no points ,,t 

 eruption or craters can be traced, or anything approaching the era- 



