Peculiarities in the Physical Geography of Otago.

Another remarkable fact in the sub-alpine part of Otago, o[gap — reason: illegible] that portion which lies eastward of the great lakes, is that most of the larger rivers do not run in what we must consider as their natural channels, but constantly cross abruptly from one valley into another. For example, the Clutha, after arriving at Cromwell, ought to have continued straight on and joined the Frazer instead of cutting through the Dunstan Range. Lake Wakatipu should have emptied itself at Kingston into the Mataura, which, in its turn, instead of breaking through the gorge at Athol, should have continued by the Dome pass to the Five-river Plains, and there have joined the Oreti. The Upper Taieri and the Idaburn should have joined the Manuhirikia at Blackstone Hill. The Lower Taieri and the Tokomairiro should empty themselves into the Clutha near Kaitangata; while the Pomahaka should have joined the Mataura instead of the Clutha.

While also the larger rivers run almost always in narrow gorge-like valleys, many of the large valleys have but very insignificent streams flowing in them. Such, for example, are the Idaburn valley, the Manuherikia valley, the Tokomairiro plains, and the Waimea plains, which last are but a portion of the valley running from the Upper Oreti to Port Molyneux. The Taieri plains also page 11do not properly belong to that river at all, for they run up the valley of the Silverstream, while the Taieri breaks suddeuly in upon them by a lateral gorge. These plains are really the valley of the Silverstream, which the Taieri has usurped.

It is these anomalies that physiography, or surface geology, undertakes to explain; and the explanation when obtained is not only curious and interesting, but it is highly important from a practical point of view; for it informs us of the successive oscillations in level that the land underwent while the surface features were being formed; and on these various oscillations of level the mode of deposition of the different river alluvia depends. But as some of these alluvia contain gold, it is evident that anything that throws light on their mode of deposition is of great importance to the alluvial gold miner.

Before, however, trying to explain the surface geology of the Province, it will be necessary to describe its geological structure, for unless this is first clearly understood, great and important errors are sure to get into our theories of the surface geology of the country.