I2 JOURNAL OF SCIENCE. 
cn 
terraces are found in all parts of the world, but in Europe and the 
British Islands they are not a prominent feature, and are generally 
rock terraces, ¢.e. terraces cut out of the solid rock, and have only a 
thin capping of shingle on the top. But with us the terraces are 
numerous, rising one behind the other, and they are formed entirely 
of silt and shingle. We have to go up into the alpine regions to find 
rock terraces. The same thing occurs in the southern and central 
portions of the North Island, although, with the exception of the 
Waikato, they are generally masked by the forest growth. In the 
north part of the North Island river terraces are a very subordinate 
feature or are altogether absent. It is my intention to-night to try 
to explain the origin of these terraces; both rock terraces and alluvial 
terraces. | 
It is plain to anyone who examines, even cursorily, these terraces, 
that they have been cut by the river, but as they differ in character 
in different countries, and are altogether absent in some, it is evident 
that the mere existence of a river and its valley is not a sufficient 
explanation. There must be some other circumstances, either in 
action now or in action at some previous period, to be taken into 
consideration and that these other circumstances must be different in 
different countries. In order to ascertain what these circumstances 
are it will be necessary to examine first the geological action of rivers 
as we now see them, and then try to deduce from these data what 
changes would take place if the conditions were to change by the 
elevation or subsidence of the land. 
Rivers are the natural drains of the land, and as the amount of | 
rainfall varies from time to time the quantity of water in rivers 
varies: in other words they are subject to floods. It is during floods 
that rivers do their work. At other times they may be said to be 
comparatively dormant. Now one inch of rain on an acre weighs 
about one hundred tons, or measures 22,400 gallons. In showery 
weather nearly one-half of the rainfall is absorbed by the soil or is 
evaporated, but in heavy rains nearly the whole runs into the rivers. 
An inch of rain is not a very large quantity to fall in a day, as much 
as six or even nine inches have been known to fall in that time in 
New Zealand. Evidently, therefore, an enormous mechanical power 
must be exerted by rivers during heavy floods. How much it would 
be useless to try to calculate. 
The rain washes the disintegrated soil into the streams and the 
streams carry it into the rivers. The amount of solid matter that can 
be held in suspension by a river depends upon its velocity. Ifthe | 
amount is increased without an increase in the volume of the water 
the velocity will be checked by the friction of the particles against 
each other and deposition will take place. We see many instances of 
this in the tailraces at alluvial diggings, which are constauily being 
filled up if too much material is put into them. But in floods the 
velocity of the river increases much more than the retardation by 
friction and therefore no deposition takes place in the riverbed until 
the water subsides. If however the river overflows its banks the 
velocity is checked and deposition of silt takes place, forming what 
are called flood-plains. The greatest retardation takes place imme- 
diately the water flows over the bank, where it suddenly gets shallow, 
and consequently the greatest deposition takes place cluse to the river 
