THE GEOLOGY OP NELSON. 
107 
and the second and subsequent acts may be coeval with the 
period of volcanic action in the Islands. 
While the tertiary sea was depositing the strata which now 
fill the valleys, and which rise in some parts to an altitude of 
2,000 feet, the higher ranges of New Zealand only were above 
water. 
Since the tertiary period, these islands have been gradually 
rising, and that rising has been coeval with the volcanic 
action, and developed to the greatest amount along the zones 
of volcanic action. It was in this time that the extensive plains 
on the east coast of the Middle Island, and the plateaus on the 
western side of the northern backbone, were raised above the 
sea. 
The best proof of this rising of the land is to be found in the 
river terraces, which strike the eye of every traveller in the 
valleys of the Wairau, Awatere, Clarence, Motueka, Wanga- 
peka, Buller, Takaka, and Aorere, and also in the lines of the 
sandy downs on the Port Cooper plains, which now, miles 
inland, mark the former limit of the sea. 
These terraces are formed by the gradual rise of the land. 
If we suppose that, while the rivers are shaping out their 
beds, the upheaving movement is intermittent, so that long 
pauses occur, during which the stream will have time to 
encroach upon one of its banks, so as to clear away and flatten 
a large space, this operation being repeated at lower levels, 
there will be several successive cliffs and terraces. It is 
remarkable that in all the valleys the cliffs of the higher 
terraces are of greater altitude than the lower. At the Buller 
river, for example, near its outlet from the Rotoiti Lake, the 
uppermost cliff is a hundred feet in height, and there can be 
distinguished in one portion of the valley not less than eight 
terraces. The character of the terraces shows that the up¬ 
heaving force has been decreasing towards the present time, 
either in power or period. The extreme height of these 
terraces, being not more than about two thousand feet up the 
valleys, shows the whole amount of rise in these islands, since 
the tertiary period, to be about two thousand feet. 
Even at the present day, there are facts which prove that 
the land of these islands is not stationary, but that the relative 
levels of water and dry land are undergoing constant modifica- 
