THE GEOLOGICAL HISTORY OF THE COLONY 437 



country during a long time. The wide distribution of 

 the striated blocks and pebbles, which are found wher- 

 ever the outcrops are sufficiently good to allow one to 

 observe the contained boulders, shows that the glacia- 

 tion was no merely local phenomenon, to be likened to 

 the very limited snow and ice covered areas within 

 tropical Africa at the present day, but that it was a 

 widespread glaciation, extending over a large part of 

 the continent north of the Karroo area. The source 

 of the Dwyka boulders has been described in an earlier 

 chapter, and we found that though the origin of many 

 is at present unknown, yet a sufficient number have 

 been recognised as having come from the Pre-Cape 

 rocks north of the Karroo to show that the main source 

 of the Dwyka series, so far as the Colonial area is con- 

 cerned, lay to the north ; the evidence hitherto noticed 

 of the movement of the ice in the northern districts is 

 to the same effect, i.e., that the ice moved southwards 

 from those districts. Whether land to the south also 

 contributed ice-borne debris is unknown. 



There is evidence from India, Australia, South 

 America and the Falkland Islands that beds of glacial 

 origin lie at the base of the rocks containing the Glos- 

 sopteris flora, just as the Dwyka tillite does in South 

 Africa ; so the cold climate which gave rise to the 

 glacial deposits affected a very large part of the earth's 

 surface during more or less the same period, though it 

 is not known that the cold climate was of equal dura- 

 tion in every part of that immense area. The explana- 

 tion of the change of climate is not clear, though much 

 has been written on the subject in recent years. 



