NA TURE 



289 



THURSDAY, JULY 27, 1905. 



THE AGEl^TS OF EARTH SCULPTURE. 

 Geology — Processes and their Results. By Thomas 

 C. Chamberlin and Rollin D. Salisbury. Pp. xi.\ + 

 654. (London: John Murray, 1905.) Price 21s. 

 net. 



IT is appropriate that this worli is written by ex- 

 perienced members of the United States Geo- 

 logical Survey who are likewise heads respectively of 

 the departments of geology and geography in the 

 L'niversity of Chicago. The main portion of the 

 volume treats of the earth's physical features and 

 their origin, and thus illustrates the forces and pro- 

 cesses which belong to the borderland between past 

 and present in which geologists and geographers are 

 alike concerned. No aspect of geology appeals to a 

 larger circle of interested students and general 

 readers. The preface being dated from the University 

 of Chicago, it may be inferred that the book is pub- 

 lished simultaneously in the United States. It is 

 printed in bold type on thick paper, and with such 

 abundant illustrations that it is a veritable picture- 

 book. There are 24 plates and 471 text illustrations ; 

 the latter are not listed, however, in the table of con- 

 tents. In the eyes of a book-lover the appearance of 

 the book is somewhat marred by its being cut down 

 rather too closely; but as the student will pay more 

 .ittention to the subject-matter he may at once be 

 assured that it is a sound, vigorouslv written work, 

 abounding in original information and suggestions, 

 and abreast of the ever-expanding knowledge to which 

 American geologists have so largely contributed. Nor 

 is there wanting due acknowledgment of many facts 

 and illustrations drawn from published sources. 



In their preliminary remarks the authors make a 

 noteworthy use of statistics. Thus we read that 

 " The total mass of the atmosphere is estimated at 

 five quadrillion ■ tons," that " .\bout 1300 quadrillion 

 tons of water lie upon the surface of the solid earth," 

 and that the volume of the stony portion is about 

 260,000 million cubic miles. These estimates, in- 

 comprehensible by themselves, are rendered useful by 

 comparisons, and the relative mass and extent of 

 atmosphere, hydrosphere, and lithosphere are thereby 

 brought clearly before the reader. It is pointed out 

 that the oceanic depressions rather than the con- 

 tinental masses are the master phenomena of the 

 earth's surface, and that if the surface were graded 

 to a common level by cutting away the land and 

 dumping the matter in the abysmal basins, the 

 average plane would lie somewhere near 9000 feet 

 below sea-level. 



In dealing with the atmosphere as a geological 

 agent, dust and blown sand, wind-ripples and wind- 

 erosion, the influence of the colour of rocks on their 

 daily range of temperature, the creep of soils and sub- 

 soils, and even the effects of lightning receive 

 attention. 



Rain and river erosion are discussed from hvpo- 

 thetical, and more fully from actual, points of view. 

 Various stages in the history of streams and valleys 

 NO. 1865, VOL. 72] 



are illustrated, and their distinguishing features in 

 vouth, in mature and in old age are described. It is 

 pointed out that the base-level of erosion and sea- 

 level are bv no means synonymous, as rivers often 

 erode below sea-level. The development of rivers 

 under different structural conditions is explained, and 

 attention is directed even to the possible influence of 

 the rotation of the earth on the erosive action of 

 streams. The beheading of one stream by another is 

 treated as " piracy," and both " foreign " and 

 " domestic piracy " are explained, the latter phrase 

 being applied to cutting off an o.\-bow in a meander- 

 ing stream. Other terms of a somewhat homely 

 nature are used, such as " scour and fill," in illus- 

 tration of the fact that a stream in flood degrades its 

 channel and aggrades (builds up) its plain. 



There is a notable chapter on ground-water, a sub- 

 ject of great scientific interest as well as practical 

 importance. The movements of ground-water include 

 the fluctuations in its upper surface or " water table," 

 and those dependent on the outflow of water in 

 springs or on its abstraction by pumping, influenced 

 as the movements also are by geological structure. 



The work of snow and ice, of continental and 

 alpine glaciers, is treated in an attractive and 

 luminous style. The way of " getting load," the 

 englacial and superglacial drift, the transfers of load 

 from basal to higher portions of the ice, and the 

 movements accompanied by shearing-planes and 

 thrusts, are duly described. 



" Hanging valleys " receive attention, and it is 

 remarked that those developed by stream-erosion are 

 not common, except in cases of the recession of a 

 waterfall past the mouth of a tributary. The features 

 are characteristic of regions recently glaciated, where, 

 as in the western mountains of North America and 

 elsewhere, a main valley has been deepened by glacial 

 action below the level of tributary streams. 



The work of the ocean is fully discussed and illus- 

 trated. The cutting of cliffs in different materials, 

 the formation of arches, stacks, and beaches, and rill- 

 marks on sands that simulate sea-weeds, and other 

 subjects large and small come under consideration. 

 The later chapters are occupied by " the origin and 

 descent of rocks "; minerals and rocks are described, 

 and some account is given of the new, and by no 

 means popular, American petrological classification 

 and nomenclature. Various structural features, cross- 

 bedding, nodules, joints, folds, &c. , as well as de- 

 formations, volcanic action, and other topics, are 

 illustrated. 



The geological functions of life are then dealt with. 

 The consumption and restoration of carbon dioxide 

 and the consequent influence on climate are discussed. 

 The agency of organisms in the disintegration of 

 rocks, and the protection they afford against erosion 

 are pointed out. Attention is also directed to the 

 influence of land vegetation on the character of sedi- 

 ments, due in the first place to the decomposition of 

 different rocks and the formation of soils — materials 

 which may be carried out to sea. On the other hand, 

 " if the surface be bare of vegetation, the crystalline 

 rocks are usually disaggregated before they are de- 



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