178 THE STORY OF THE EARTH AND MAN. 



But in eacli of the great successive heaves or pul- 

 sations of the PalaBOzoic earth, there was a growing 

 balance in favour of the land as compared with the 

 water. In each successive movement more and more 

 elevated land was thrown up, until the Permian flexures 

 finally fixed the forms of our continents. This may be 

 made evident to the eye in a series of curves, as in the 

 following diagram, in which I have endeavoured to 

 show the recurrence of similar conditions in each of 

 the great periods of the Palaeozoic, and thus their 

 equivalency to each other as cycles of the earth's 

 history. 



There is thus in these great continental changes a 

 law of recurrence and a law of progress ; but as to the 

 efiicient causes of the phenomena we have as yet little 

 information. Ifc seems that original fractures and 

 shrinkages of the crust were concerned in forming the 

 continental areas at first. Once formed, unequal 

 burdening of the earth's still plastic mass by deposits 

 of sediment in the waters, and unequal expansion by 

 the heating and crystallization of immense thicknesses 

 of the sediment, may have done the rest; but the re- 

 sults are surprisingly regular to be produced by such 

 causes. We shall also find that similar cycles can be 

 observed in the geological ages which succeeded the 

 Palaeozoic. Geologists have hitherto for the most part 

 been content to assign these movements to causes 

 purely terrestrial ; but it is difiicult to avoid the sus- 

 picion that the succession of geological cycles must 

 have depended on some recurring astronomical force 



