The point of difficulty in the behaviour of the tides is this. The water is running rapidly out of the lake for nine hours; whilst during the remaining three hours there is a sluggish return-flow up the passage into the lake. A far greater quantity of water finds an exit by the passage than is returned by the same channel; and I can only explain this by assuming that there is an extensive percolation of water from Natewa Bay into the lake. It is easy to show that with such a narrow effluent, which cannot have a sectional area exceeding 180 square feet, the level of the lake would be only lowered 2 or 3 feet, if the average velocity during the nine hours was two nautical miles. The great bulk of the water would thus remain unchanged. The ultimate result of such conditions would be a lake of brine. Since, however, the sea-water of the lake possesses the ordinary density, it is apparent for this reason only that there is some other means of supply than by the present narrow passage leading to the sea. The mean level of the Salt Lake is evidently rather above that of the sea, perhaps a foot or two; and the “rise-and-fall” is probably very small.
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES (_continued_)
THIS remarkable peninsula is connected with the rest of the island by the low-lying Salt Lake district, a narrow isthmus, described in the preceding chapter, which one can cross without rising 50 feet above the sea. My acquaintance with this region is far from complete; but from the following notes a fair general idea of its geological characters may be gathered.
By referring to the map it will be seen that there are three groups of mountains. The north-eastern culminates in Mount Freeland or Ngala, 2,740 feet; the southern is formed by the rugged Waikawa Range, 1,540 feet; whilst the Lea Range to the west attains in Ngalau-levu a height of 1,960 feet. They are much cut up by the denuding agencies, and all bear the stamp of an ancient land-surface. Though hot springs are not infrequent, as at Ndreke-ni-wai, Waikatakata, Ndevo, and Navuni, no evidence of recent volcanic action came under my notice. Submarine deposits occur at intervals on the surface up to elevations of 1,000 feet and over; but with the exception of the comparatively recent upheaval or emergence of some 20 or 30 feet, indicated by the raised reef-masses and foraminiferous tuffs and clays in different parts of the coast, there is nothing to suggest that these changes did not occur ages since. In the frequent alteration of its andesitic rocks, and in the occasional occurrence of porphyrites, we have sufficient indication of the antiquity of this part of the island as far as its volcanic history is concerned.