34 PRESIDENTIAL ADDRESS. SECTION B. 



taceous or Tertiary times, but it apparently gave rise to no 

 superficial flows that have survived to the present day. 



There are several striking points of difference between 

 this history of vulcanism in South Africa and that recorded in 

 the northern hemisphere. Setting aside possible equivalents 

 of the earlier Palaeozoic volcanic rocks of Europe and North 

 America, we find nothing to compare with the later Palaeozoic 

 and early Mesozoic volcanic rocks of those regions, nor again 

 with the late Cretaceous and Teritary flows of India, nor with 

 the immense Tertiary flows of north-west Europe and western 

 America. 



It is by no means evident that South Africa was the scene of 

 more intense or more wide-spread vulcanicity in early times 

 than in later, and it seems very probable that we have no 

 record of greater activity than that which still persists in the 

 Hawaiian island, Iceland and the Central States of America. 

 It is not alone the positive evidence which bears out this 

 opinion; the negative is of almost equal importance; for the 

 great thicknesses of ancient rocks in the Malmesbury, Ibiquas 

 and Cango beds in the south and west, and in the Witwaters- 

 rand beds of the north, have not yet yielded definite evidences 

 of contemporaneous igneous rock.^. 



South Africa brings no support to the view that the earth's 

 volcanic activity has declined, steadily or irregularly, from the 

 earliest times of which we have knowledge; the present 

 quiescence in this region may be merely temporary; it has cer- 

 tainly not endured so long as the period referred to abovj|^ 

 which lasted from Silurian to Jurassic times. 



Of the varieties of climate which affect the earth's surface 

 to-day none leaves such characteristic traces as a glacial 

 climate. Tropical, temperate and desert conditions are accom- 

 panied each by its characteristic processes of change at the 

 surface, but though these are becoming better understood 

 year by year, yet the results of this knowledge can at present 

 only be applied with difficulty and to a very limited extent to 

 the stratified rocks, partly because ancient land surfaces and 

 the deposits formed on them have rarely been preserved. But 

 a glacial climate leaves more enduring marks, both on the up- 

 land surface and in the deposits formed on low ground and 

 near the coast. 



No satisfactory explanation other than that afforded by the 

 assumption of glacial conditions has been put forward to ex- 

 plain the peculiarities of such rocks as the boulder beds of the 

 Dwyka series. Many years passed between the first recogni- 

 tion of the meaning of these rocks in India and South Africa 

 and the general acceptance of the evidence by European and 

 American geologists; even as lately as 1906 the opinion was 

 expressed in a geological treatise of the first rank that 

 " in fact all proof of a glacial temperature is wanting in the ancient periods 

 of the globe, and in spite of the complaisance with which several geologists 

 have admitted .... the existence of Silurian or even Cambrian 

 glaciers, we do not believe that any naturalist subscribes to an hypotheses 

 in formal discord alike with the conditions of relief in those days and the 

 existence of corals in all the Palaeozoic seas." 



