l6o STUDIES FOR STUDENTS 



which envelope them; each of major importance in controlling the 

 character of the lands and the evolution of their inhabitants. 



The relations of the lands and seas of previous ages is the one 

 of these environmental conditions which has been longest under 

 investigation, since the seas have left a positive record of their inva- 

 sions in the form of marine sedimentary strata and the remains of 

 the inhabiting organisms. 



The topographic changes of the lands of earlier ages give prob- 

 lems of equal importance, both in connection with the history of 

 crustal movements, as sources of the sedimentary rocks, and in the 

 evolution of terrestrial life; now raising mountain barriers and 

 again opening fields for migration and expansive evolution. But 

 until recent years the problem of restoring earlier land-forms had not 

 attained to a systematic and scientific state. That knowledge of 

 the topographic history of the land should be slower in development 

 than the relations of land and sea is seen to be natural when it is 

 considered that each erosion surface is made only through the de- 

 struction of the preceding land-form, with the result that the earlier 

 leaves in the volume of land history, contrary to that of the sea, are 

 destroyed in the making of the last, and it is only through processes 

 of deduction that the earlier stages may in a general way be restored. 



The third great problem of terrestrial environment, the succession 

 of ancient climates, lags still farther behind in development, but is 

 no less important in a complete understanding of the history of the 

 earth and its inhabitants. This lack of development is doubtless 

 due to the intangible nature of climate and the absence of direct 

 record of its geologic changes. When it is considered, however, how 

 fundamental are the relations of continental deposits to the climates 

 in which they are formed, it is seen that the record of geologic climates, 

 while indirect and largely awaiting interpretation, is nevertheless 

 in existence. 



The significance of salt and gypsum deposits on the one hand or 

 of glacial deposits on the other is of course universally recognized, 

 but these are the marks of climatic extremes. The causes of climatic 

 variations, as distinct from the record, have likewise in recent years 

 received a great amount of attention, with the result that a considerable 

 body of knowledge has taken the place of previous speculation. But 



