Page:Transactions and Proceedings of the New Zealand Institute - Volume 1 (2nd ed.).djvu/191

157 communicating the following essays, I will take the opportunity of explaining briefly, by a single example, the chief physical peculiarities which regulate the distribution of the vegetation in the South Island.

The accompanying diagram (pl. V.) is an ideal section across the island, between latitudes 40° 30' and 46° 30'S. The greatest altitude met with in such a section will be 10,000 feet, but the mean elevation of the ridges that connect the various summits is barely 5,000 feet, while in these occur breaks, or “passes,” in the mountain chain, which, by permitting the passage of the western winds, give rise to local modifications of the flora at the points where they lead out on the eastern slope. The best known of these are the pass from the head of the Wanaka Lake, by which Dr. Haast crossed to Jackson Bay; and the Greenstone Pass, leading from the Wakatipu Lake to Martin Bay. Another pass, only a few miles in length, crosses the narrowest part of the Southern Alps, between the head of the sounds and arms of Te Anau Lake.

These breaks in the mountain chain have all about the same altitude of less than 2,000 feet above the sea level, which is sufficiently low to admit of the transfusion of many species of plants.

From the fact that these passes follow longitudinal valleys with a succession of short gorges at right angles to their general course, and do not coincide with straight transverse depressions, the influence which in the latter case they would have exercised on the climate of the interior is greatly reduced; nevertheless, the mild and genial climate that is experienced in the neighbourhood of the Wanaka and Wakatipu Lakes, is to be attributed, in a great measure, to the existence of these deeply-cut notches in the mountains.