Igg ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



mals and plants migrate from place to place, the time occupied by 

 the migrations of suitable forms is so negligible when compared with 

 the length of the chapters of geological history that their fossil 

 remains have proved to be the best means for correlating strata 

 over broad stretches of the earth's surface. This correlation has 

 converted the fragments of local history thus revealed into at least 

 the outlines of the geological story of the world. 



It was not till 1885, however, that the accumulation of data of 

 this type was sufficient to enable the great geologist, Suess, an 

 Austrian but born in this country, to assemble and correlate them, 

 and to deduce from them further principles which have been the 

 mainstay and inspiration of his successors. We owe to Hertha 

 Sollas and her father the rendering of this great work, The Face of 

 the Earth, into English; and to Emmanuel de Margerie and his 

 colleagues a French translation enriched with a magnificent series 

 of maps and sections such as could only have been brought together 

 by one with the most remarkable bibliographic knowledge; a veri- 

 table recension of the original. 



The nature and associations and the distribution in time and space 

 of modern changes in the relative levels of land and sea, as detected 

 at sea margins and by altitude survey, and of older changes betrayed 

 by such evidence as submerged forests and raised beaches, had con- 

 vinced geologists that the unstable element was not the fickle and 

 mobile sea, but the solid if elastic earth crust. They naturally ap- 

 plied the same explanation to those encroachmeMs of the sea in 

 the past which had resulted in the formation of our stratified rocks. 

 But while some investigators were content with one form of move- 

 ment — that due to lateral pressure — to explain both the formation 

 of mountains and the rise and fall of the land, others called in a 

 different cause for the latter. Without entering into a discussion of 

 causes it may be well for us to distinguish the orogenic or mountain- 

 forming from the epeirogenic or continental movement. 



The evidence collected by Suess proved that these last great land 

 and sea changes had occurred simultaneously over whole continents 

 or even wider regions. Such great submergences as those to which 

 the Cambrian rocks, the Oxford clay, and the chalk are due were 

 of this character; while, in between, there came times of broad ex- 

 pansions of continental land and regressions of the sea. These 

 changes were in his view on far too grand a scale to be compared 

 with, or explained by, the trivial upheavals and depressions of land 

 margins of the present day, which he showed could mostly be cor- 

 related with volcanoes or earthquakes, or with such incidents as the 

 imposition or relief of ice sheets on an elastic crust in connection with 

 glacial conditions. 



