786 



SCIENCE. 



LN. S. Vol. XIX. No. 490. 



hanging valleys, presented by Gilbert, although 

 seemingly complete and unassailable, is per- 

 haps not final. 



In the volume under review the term ' tidal 

 glacier ' is substituted for the previously used 

 and longer term ' tide-water glacier,' to desig- 

 nate ice streams which end in the sea. This 

 substitution is welcome on account of its 

 greater brevity, but I doubt if its meaning will 

 bo as readily grasped by the general reader as 

 in the case of the longer term. 



In the sections presented by Gilbert of the 

 ends of tidal glaciers, the ice is represented 

 as overhanging, and the only mode of forma- 

 tion of bergs that is recognized is from the 

 breaking away of exposed portions of an ice 

 cliif and their fall into the water at its base. 

 The view is favored that the sea water causes 

 a more rapid melting of the submerged por- 

 tion of a glacier which advances into it, than 

 occurs on the part of the ice above water. 

 There can be no difference of opinion in this 

 connection so far as the formation of most 

 bergs from the glaciers of Alaska is concerned, 

 but the ' blue bergs ' which occasionally rise 

 to the surface of the water at a distance of 

 several hundred, and as estimates indicate, 

 fully a thousand feet in some instances, in 

 front of the visible ice-cliff of a tidal glacier, 

 certainly indicate exceptional conditions. The 

 birth of bergs from the extended submerged 

 extremity of a glacier does not seem to have 

 been observed during the Harriman Expedi- 

 tion. 



In a chapter bearing the caption, ' General 

 Considerations as to Glaciers,' four groups of 

 features are discussed. (1) The broader char- 

 acters of the surfaces of the lower portion of 

 a glacier, or the ' glacier proper,' such as its 

 evenness of surface as compared with the con- 

 tours of the rock-floor on which it rests ; lateral 

 ice-cliffs due to heat reflected from the adja- 

 cent land and the influence of margining 

 streams ; and ' crevass cycle,' or the sequence of 

 changes a series of crevasses, such as forms be- 

 low an ice cascade, pass through. (2) Glacial 

 sculpture, in the consideration of which the 

 conspicuously different results produced by 

 abrasion and ' plucking ' are described and 

 illustrated by photographs and sketches. (3) 



The pressure and erosive power of tidal 

 glaciers, in the consideration of which an 

 interesting theoretical discussion is indulged 

 in in reference to the manner in which the 

 distal end of a tidal glacier is suijported. (4) 

 The resemblances, differences, homologies and 

 analogies of streams of water and glaciers, 

 are clearly and most instructively indicated. 



As I have attempted to show, Gilbert's 

 sumptuous volume is most welcome to ,the 

 special student of glaciers, both on account of 

 the new facts it contains and of the new 

 thoughts presented. It is clearly and con- 

 cisely written, and must appeal to the general 

 reader and entice him to make still more 

 arduous journeys through the fields of glacial 

 literature. One of its chief values is as a 

 reading lesson for advanced students in col- 

 lege classes. 



Geology and Paleontology. — Volume IV. of 

 the Harriman Alaska Expedition bears in- 

 ternal evidence of being based on a reconnais- 

 sance much more clearly than its companion 

 volume on glaciers and glaciation. Its table 

 of contents reads : ' Introduction,' by G. K. 

 Gilbert ; ' General Geology,' by B. K. Emer- 

 son ; ' The Alaska-Treadwell Mine,' ' Geology 

 about Chichagof Cove,' and ' Minerals,' by 

 Charles Palache ; ' Mesozoic Invertebrate Fos- 

 sils,' by William H. Dall ; ' Fossils and Age 

 of the Yakutat Formation,' by E. 0. Ulrich 

 and ' Fossil Plants from Kukak Bay,' by F. 

 H. Knowlton. 



A noteworthy result of the geological 

 studies, as is stated by Gilbert in the intro- 

 ductory chapter, is the correlation on fossil 

 evidence of slates and shales in three widely 

 separated localities — Yakutat Bay, Prince 

 William Sound and Kadiak Island — and the 

 detei'mination of their age as Jurassic. The 

 correlation referred to of terranes in areas 

 over 500 miles apart, as stated by Ulrich, ' is 

 by means of a fossil species of definite char- 

 acter, Terehellina palachei, common to them 

 all.' This fossil is the type of a new genus of 

 burrowing worm, of which only the tubes it 

 made, composed of cemented sand-grains, have 

 been discovered. The assig-nment of a Liassie 

 (Lower Jurassic) age is based mainly on four 

 species of algre belonging to two genera which 



