180 Proceeclinfjs of the llotjcd Physical Society. 



down of the earth's crust within the oceanic basins — if the 

 continents have been squeezed up by the tangential thrusts 

 exerted by the sinking areas that surround them — then it 

 follows that while lands have been gradually extending over 

 the continental plateau, the bed of the ocean has been 

 sinking to greater and greater depths. 



If this general conclusion holds good, it is obvious that the 

 oceanic troughs of early geological times could not have been 

 so deep as they are now. During the Palaeozoic period, the 

 most continuous areas of dry land, as we have seen, were 

 distributed over the northern parts of our hemisphere, while, 

 further south, groups of islands indicated the continuation of 

 the continental plateau. Doubtless South America, Africa, 

 Asia, and Australia were, at that distant date, represented by 

 similar detached areas of dry land. In a word, the primeval 

 continental plateau was still largely under water. Judging 

 from the character and broad distribution of the Palaeozoic 

 marine faunas, the temperature of the sea was wonderfully 

 uniform. There is certainly nothing to indicate the existence 

 of such climatic zones as those of the present. We know 

 very little of the terrestrial life of early Palaeozoic times — 

 the Cambro-Silurian strata are essentially marine. Land- 

 plants, however, become more numerous in the Old Eed 

 Sandstone, and, as every one knows, they abound in the 

 succeeding Carboniferous and Permian systems. And the 

 testimony of these floras, points to the same conclusion as 

 that furnished by the marine faunas. The Carboniferous 

 floras of the Arctic regions, and of temperate Europe and 

 America, not only have the same fades, but a considerable 

 number of the species are common to both areas ; while 

 many European species occur in the Carboniferous strata of 

 Australia and other distant lands. This common /aae6', and 

 the presence of numerous cosmopolitan forms, surely indicate 

 the former prevalence of remarkably uniform climatic con- 

 ditions. The conditions, of course, need not — indeed, could 

 not — have been absolutely uniform. At present the various 

 climates which our globe experiences depend upon the 

 amount of heat received directly and indirectly from the 

 sun — oceanic and aerial currents everywhere modifying the 



