PRESIDENT S ADDRESS. 



for the kind of climate, not to attempt deductions from any- 

 assumed physical changes, such, for instance, as a different distri- 

 bution of land and water. Such deductions may well be profit- 

 able in the future when there is a more secure foundation to 

 build on; when, for instance, at least the main features of the 

 hypothesis of crustal creep put forward by Taylor and Wegener- 

 shall have been settled, according to which the existing continents 

 have changed their positions in latitude and longitude very con- 

 siderably during past times. It is obvious that should the 

 hypothesis be found valid it will bear intimateh' upon climate, 

 and it was used by Dr. du Toit at our last Annual JNIeeting^ to 

 explain the hitherto insoluble difficulty of understanding the 

 Carboniferous glaciation in present-day temperate latitudes of the 

 southern hemisphere. Revolutionary as the hypothesis may- 

 appear, it is now assured of serious attention, and it is on 

 geological evidence that it will stand or fall. In our time we 

 have seen conclusions about the age of the Earth drawn from 

 geological and biological arguments receive support from the 

 successors of astronomers and physicists who, reasoning fifty years 

 ago from too limited knowledge, said that evolutionists' demands 

 upon time were much too great ; so whatever may be the physical 

 difficulties of the new hypothesis, it will be judged by such 

 methods as close comparison of the rocks and structure of eastern 

 South America and western Africa. 



The evidence upon which we have to depend for recognising 

 kinds of former climate falls into four groups ; the first is drawn 

 from the characters of the rocks formed during the period con- 

 cerned; the second relates to the shapes of the land surface 

 resulting from the long duration of more or less constant climatic 

 conditions; the third concerns the distribution of animals and 

 plants; and the fourth is historical, the records, archseological, 

 traditional and written, of the period since man occupied the 

 country. It is obvious that these four groups are very dissimilar, 

 and, in general, each can be drawn upon for information about 

 certain periods only. 



LiTiioLOGicAL Evidence. 



The lithological evidence is by far the most important for 

 all but comparatively recent times, certainly for the whole of pre- 

 Tertiary time. The character of sediments formed by the accumu- 

 lating materials worn from the land, depends upon the kind of 

 rock forming the land and the climate prevailing there. Though 

 the Earth's crust is made up of a large number of different 

 rocks, their mineral constituents are few; the felspars, quartz, 

 mica, and a few other dark silicates, are by far the most im- 

 portant, and they, together with water, oxygen, and carbon 

 dioxide obtained from the air in the course of weathering, form 

 99 per cent, of the sedimentary rocks. The grains and much of 

 the cement of the sediments ultimately come from igneous 

 rocks, which, in spite of their name, have nothing to do with 

 fire but are mixtures of minerals which crvstallised out from 



