418 TransactioJis . — Geology. 



lived are left behind, and by repeated accumulations of such 

 shells vast deposits of calcareous rock are formed. All our 

 great masses of limestone have been formed in this way, the 

 coral polypi having had the greatest share in their formation. 

 Try for a short time, then, to blot New Zealand as it is out of 

 your memory, and conceive instead a coral reef in a tropical 

 sea— something like the Great Barrier Eeef off the coast of 

 Australia. The sea-bottom is slowly sinking, but fresh layers 

 of coral are formed, till several hundreds of feet have been pro- 

 duced. Then, owing to rapid subsidence or change of climate 

 (or, perhaps, both), the work of coral-forming ceases, and layer 

 upon layer of fine mud is deposited on the top of the coral- 

 reef, till a thickness of several hundreds of feet of fine silt has 

 been formed. This silt, of course, must have come into the 

 ocean from the rivers of some adjacent land-area, and repre- 

 sents so much waste from that land. While this silt was 

 being deposited the superincumbent pressure of its own 

 weight, added to the weight of the ocean above, would con- 

 solidate it into a hard rock, and produce what is known 

 as a clay-slate. Then an upward movement commences in 

 the earth's crust beneath these rocks, and they are gradually 

 raised till they stand thousands of feet above the level of 

 the sea — not as we know them now, presenting an innumer- 

 able variety of landscape, made of valleys, and mountain- 

 ridges, and mountain-peaks, but as a broad belt of elevated 

 land. After a mountain-chain had been thus formed — or, 

 probably, during the latter period of its formation — the 

 pressure from beneath was so great that the overlying crust 

 of limestone and slate gave way, and rock-matter, in a more 

 or less plastic state, from the interior of the earth, was forced 

 into the gap, thus giving rise to the mineral belt, w^hich, as I 

 have already stated, is composed almost entirely of crystalline 

 rocks. 



After the formation of the mountain-chain as a broad belt 

 of elevated land, the work of denudation went steadily for- 

 ward. The heat of the sun by day, the cold of frosts by night, 

 the storms of rain, and the never-ceasing chemical action of 

 the atmosphere began to soften and wear away the rocks. 

 The rain-water, in its endeavour to reach the sea, would form 

 watercourses, which, by the erosive action of the water, would 

 deepen and widen their channels till rivers were formed. The 

 rivers, especially in time of flood, aided by innumerable frag- 

 ments of rock loosened from the parent rock in the manner 

 already described, would continue to wear down the country, 

 thus forming the broad and deep valleys so characteristic of 

 the hilly parts of this district. "When looking down into any 

 of these valleys from some elevated spot, and remembering at 

 the same time that the whole of the valley has been scooped 



