January 31, 1896.] 



SCIENCE. 



169 



all attempts to write the physical history of 

 North America, with which the book is mainly 

 concerned, are the reports of the geological sur- 

 veys of Canada, the United States, and of many 

 individual States. The results of these great 

 surveys reach the people and the schoolroom < 

 directly, to only a comparatively limited extent; 

 probably their greatest popular usefulness lies 

 in the fact that they are mines of wealth to 

 those who attempt to popularize and disseminate 

 scientific knowledge. 



Physical geography is treated by Prof. Tarr 

 under three leading topics : The Air, The Ocean, 

 The Land. 



The Air : The part treating of the air begins 

 with an account of the relations of the earth to 

 other members of the solar system and is in fact 

 an introduction to the entire subject of physical 

 geography. This chapter probably differs less 

 than any other portion of the book from older 

 treatises on the same subject. Necessarily the 

 subject-matter to a great extent is borrowed 

 from astronomy. 



The discussion of atmospheric temperatures, 

 moisture, condensation, clouds, etc., the nature 

 and origin of storms, distribution and character- 

 istics of climate and other similar phenomena, 

 brings out the results of the most recent studies 

 in this important branch of the subject. To a 

 great extent these chapters are a compend of 

 Davis' Meteorology, a book that should be at 

 hand when instruction in this portion of the 

 subject is given. 



The Ocean : In dealing with the geography of 

 the sea, the rich store of knowledge resulting 

 from the Challenger, and other similar expedi- 

 tions, furnishes the data for presenting a com- 

 prehensive outline of the results of recent sur- 

 veys. Some of the subdivisions of the subject 

 as treated are : methods of deep-sea explora- 

 tions ; topography of the sea bottom and of 

 coast lines ; deposits now forming on the sea 

 floor ; temperatures ; chemical composition, 

 circulation, etc., of sea waters; general distri- 

 bution of life in the sea ; the causes of currents 

 and tides ; and the effect of the movements of 

 sea water are discussed and illustrated by dia- 

 grams, maps and photogi-aphs. 



The Land : It is in this portion of the book 

 that the greatest advances, both in geography 



as a science and in methods of study, are shown. 

 The processes by which the rocks forming the 

 land are disintegrated and carried away are 

 discussed and the resulting changes in topog- 

 raphy clearly described. The fact that all 

 rocks which rise above sea level are constantly 

 yielding to chemical and mechanical agencies 

 and being removed by streams in solution and 

 suspension leads to the recognition of a funda- 

 mental principle, first definitely stated by Major 

 Powell, which is of wide application in both 

 geography and geology. Tliis tendency to re- 

 duce all land areas to the level of the sea, or to 

 baselevel, as it is termed, if not counteracted by 

 movements of elevation, will result in the pro- 

 duction of plains. Such plains of subajrial de- 

 nudation, or peneplains, are a characteristic 

 feature of many regions. 



A knowledge of the way in which streams 

 deepen and broaden their valleys, and slowly 

 adjust themselves to rock structure, gives 

 meaning to a multitude of geographic forms, 

 that would otherwise appeal to the eye alone 

 without awakening a mental picture of the long 

 series of changes of which they are the result. 



The deposition of the waste of the land in 

 flood plains and deltas, and its distribution over 

 the bottoms of lakes and on the ocean's floor, 

 illustrates other phases of the never-ending 

 changes that attract the eye of the geographer. 

 These wide reaching processes and the character 

 of the results they produce are tersely outlined. 



The characteristics of glaciers and the changes 

 they bring about in the topography of the land, 

 both by erosion and deposition, form a chapter 

 that cannot fail to awaken interest especially in 

 the minds of students whose homes are in the 

 northeastern States or Canada, since not only 

 the general expression but almost every detail 

 in the landscape with which they are familiar 

 is an inheritance from an ice invasion. 



The study of coast lines shows that the agen- 

 cies by which the relief of the surface of the 

 land is modified are supplemented by analogous 

 agencies which are constantly altering the di- 

 rection and varying the details of the margins 

 of continents and islands. 



Many of the results of erosion and deposition 

 are illustrated by home example and supple- 

 mented by photographs of American scenery. 



