164 THE VICTORIAN NATURALIST. 



and the platform, as it were, of this material rises to just above 

 sea-level. On this rests a Tertiary formation of dune sand very 

 similar to what is found about Sorrento. This has been derived 

 from some old sea-shore where an ancient sea pounded up shells 

 with the grains of quartz and felspar derived from the breaking 

 down of the granite, and strong winds took the smaller pieces 

 away and piled them up high and dry. The prevalent winds are 

 from the west and south-west, and have been so for ages, for the dune 

 sand (which had its origin on the west side only) now completely 

 covers the granite platform, and is advancing into the sea on the 

 eastern side of the island. The base rock, as would be expected 

 from this, is exposed at the seashore for practically the whole 

 length of the west coast, and at any part its hard and varied 

 character can be seen. Much of it shows gneissic characters ; 

 it is in layers, or bedded, as if it underwent movement before 

 thoroughly cooled. The gneiss gives an indescribably wild and 

 rugged coast-line standing out in many hard ridges and sharp 

 points, cleft by deep gulches, and guarded by many outlying reefs. 

 In great contrast is the purely granite coast where beautifully 

 rounded and tumbled boulders are found. To see what variety 

 there is in the graintoid material, one has only to examine perhaps 

 loo yards of the coast where the foliated rock appears. Beds of 

 gneiss, very fine in the grain, are commonest, but between them 

 will be found layers of coarse admixtures of quartz and mica, of 

 quartz and felspar, and of quartz and felspar with hemihedral 

 crystals of tourmaline, some fine like pins and others the thick- 

 ness of a finger, while thrust up between or into these beds are 

 bosses of granite and dykes of a very fine-grained rock. 



The platform of base rock, however, on the west coast averages 

 only lo feet above sea-level, though at one place inland it rises 

 into a hill loo feet or more. In some low-lying places the 

 granite is decomposed to a gritty clay, from which bricks have 

 been made. The influence of this platform on the drainage 

 water can easily be imagined. It forms an impassable barrier, 

 effectually preventing it soaking away to unknown depths. Thus 

 lagoons in plenty are formed in low-lying places by the under- 

 ground waters soaking in laterally through the loose sand dunes. 

 In two or three specially favoured places the granite outcropping 

 around the margins proves them to be natural reservoirs set in 

 exposed depressions of the bed rock. Then, at certain points, 

 the overflow from the lagoons filters away and finds outlet to the 

 sea, sometimes by running streams, but more frequently by per- 

 colating beneath the sand dunes again and trickling out over the 

 granite just above high water mark in many clear springs. The 

 largest sheet of water is in the north, and well worthy of the 

 name of Big Lake, for it is 700 acres in extent and of consider- 

 able depth. 



