MATTHEW, CLIMATE AND EVOLUTION 175 



dependent upon the known facts in regard to isostasy. The rocks under- 

 lying the oceans are heavier than those underlying the continents, as is 

 proved by the deficiency of gravity measurements in the continents as 

 compared with those in oceanic areas, the deficiency being most marked 

 in certain, mostly high-lying parts of the continents. The conclusion 

 appears unavoidable that in a broad way the present distribution of land 

 and shallow water on the one hand, of deep water on the other, has been 

 substantially unchanged.'* Changes in past geography have been of two 

 kinds : 



1) The coutinents have been alternately partly overflowed and then have 

 emerged to the limits of the continental shelf. 



2) Certain lines of unstable conditions have been subject to folding and 

 crumpling, accompanied with great changes of level. 



DISTRIEUTIOK OF LAND AND "WATER, PRESENT AND PAST 



The present distribution of land and water shows the great land masses 

 located mostly in the northern hemisphere.^ The land areas, extended 

 to the borders of the continental shelf, form a single great irregular mass 

 with three great projections, South America, Africa and Australasia, 

 radiating out from it into the southern hemisphere. A rise of 600 feet 

 would unite all the land into a single mass." Only New Zealand, Mada- 

 gascar, the Antilles and numerous small oceanic islands would remain 

 separate. The East Indian islands would be part of the main land. A 

 lowering of 600 feet would isolate North America, South America, Asia, 

 Africa and Australia as separate insular continents. Europe would 

 form a complex of islands and peninsulas much like the East Indies of 

 to-day. 



According to the present theory, we have recently passed through an 

 epoch of maximum continental extension and zonal climate culminating 

 in the Glacial age, marked by great aridity in the equatorial zones, by 

 cold and glaciation towards the poles and in high mountain regions. A 

 much earlier extreme of aridity and glaciation is seen in the Permian," 

 and less marked extremes at the end of the Trias and at the besrinnino- 

 and end of the Cretaceous. The alternate extremes of warm moist and 



<lu this connection, tiowever, the suggestion of Bailey Willis that the present isostatic 

 compensation may be unusually complete must be borne in mind. 



' It should be observed that the Antarctic continent, according to the latest data avail- 

 able, equals or exceeds any of the other continents in bulk of emerged land : but it is sur- 

 rounded by deep oceans of vast extent. 



« Australia forms a doubtful exception. The soundings in the Indo-Australasian 

 region are insufficient to determine with certainty whether or not there is any continu- 

 ous bridge within the 100-fathom line. 



■'The earlier Paleozoic extremes of aridity — Cambrian and Devonian — do not come 

 within the scope of this discussion. 



