Hill. — Geology of Hawke's Bay. 473 



the excess of cold in the annual balance between the heat of 

 the sun above and below the snow-line ; but the heat below 

 the snow-line may be equal to the maintenance of a warm, 

 temperate flora, and yet be insufficient to melt the large 

 accumulation of glacial ice from an area beyond the snow- 

 line favourable to deposition and accumulation such as must 

 have existed in order to produce glaciers like those which are 

 supposed to have covered north-western Europe and a large 

 portion of America, north of the parallel of 50° N. 



The warm temperate flora of our own country in the midst 

 of glaciers which descend to within 703ft. of sea-level shows 

 that under favourable conditions glaciers can exist in close 

 proximity to a rich and varied flora, capable of maintaining not 

 merely an abundance but also a variety of animal life. But the 

 seasonal contrasts such as prevailed in glaciated districts may 

 have been even wider than those now prevailing within a given 

 vertical and horizontal area, if we suppose, as is very probable, 

 that the water- and land-areas were very different from what 

 they now are. The surface of Europe shows that great earth- 

 movements must have taken place during the later Tertiary 

 period, and these movements were necessarily accompanied 

 by the disappearance of some and the appearance of other 

 river-basins. This fact, it appears to me, is too much over- 

 looked at the present time by geologists. It is too often 

 forgotten that denudations, aerial, aqueous, and seolian, are 

 constantly modifying and destroying river-basins and bring- 

 ing into being new ones ; so that at the close of a period 

 the differentiations may be supposed to have reached their 

 maximum, there being numerous river-basins of small size 

 separated from one another by low elevations ; whilst the 

 commencement of a period sees few rivers, of mighty size, the 

 drainage of a single river embracing what is now understood as 

 a "river system." This latter condition is, no doubt, what 

 existed at the beginning of the period known as the Pleis- 

 tocene, when the physical changes were such as to produce 

 wide contrasts within the same area of vertical and horizontal 

 space. 



And that these contrasts existed seems to me assured 

 if we consider what great varieties of aerial pressure are 

 possible on the earth's surface at the present time in conse- 

 quence of the inequalities of surface. First, let it be assumed 

 that the land-area covering the earth is one-fourth that of the 

 water-area. The average height of this surface above sea- 

 level is 2,660ft. At sea-level the pressure of the atmosphere 

 is on the average 151b. to the square inch, which represents 

 31in. of barometric pressure. At 2,660ft. the pressure of the 

 barometer is 28in., which corresponds to 13-51b. to the square 

 inch. By the graphic method this would give an average 



