402 Geology of 



lessened their fall, although still haying the character of true- 

 torrents, descending in their course at a more uniform but lesser rate 

 than the post-pliocene fans of the former larger rivers. They have, in 

 consequence, according to their present size and position of sources, 

 cut more or less deeply into these deposits, hut reach a point where 

 they intersect the line, below which the old fan falls more considerably, 

 and thus the present rivers, instead of excavating, begin to fill up, 

 raising their beds above the older deposits and forming new fans, 

 The present rivers repeat simply on a smaller scale the action of the 

 large post-pliocene torrents forming the upper plains ; they ako 

 debouch from a gorge, although the sides of it consist only of shingle, 

 and spread fan-like over the lower grounds, the axis of the course of 

 the river changing so as to build it up regularly. Consequently it has 

 been found by actual survey, that the contours taken at an equal 

 distance from the beginning, or perhaps better stated, from the 

 emergence from the older beds, have an equal gradient and altitude. 

 Thus, as before said, the higher this point lies above the junction of 

 the river with the sea, the larger will be the radius over which the 

 river can roam and raise its bed. It was, therefore, of the greatest 

 value to the district to ascertain not only the exact spot where the 

 recent accumulations begin to spread over the older ones, but also to 

 find, by careful measurements, what is the ratio of the present rise 

 above the level of the sea, as well as above a given spot of the plains 

 of older origin. The fact, however, that only the Waimakariri, and in 

 a minor degree the Rangitata on its southern and the Rakaia on its 

 northern banks, for a small extent are subject to such an occurrence, 

 is worth recording, and shows distinctly that the two last-mentioned 

 rivers flow on the sides of the large post-pliocene deposits described 

 previously. Here some rich alluvial land is situated, and thus those 

 small strips, endangered by the aberrations of these rivers, are of 

 far greater importance than a look at the map at first might suggest. 



In the following four tables the data collected during a number of 

 years, as to the fall of the plains, and the rivers which formerly built 

 them up, are given. They are taken from my Report on the formation 

 of the Canterbury plains, to which a number of details obtained since 

 and also some corrections have been added. 



