Geological Work of Frost and Fire. 11 



a warm dark column rises up like tlie spirit whom the Arabian 



fisherman let out of tlie copper vase When the 



water is hot, a thing like a round-headed mushroom grows 

 rapidly out of the neck, and takes all manner of strange 

 shapes." 



Experiments of this kind admit of indefinite variation. 

 They can be performed with trifling expense, and are excel- 

 lently adapted to explain the nature of actions by which climate 

 is affected, and the superficial strata of the earth exposed to 

 influences that modify their form. 



Mr. Campbell's illustrations of " river marks" are very 

 instructive, but he chiefly notices the violent action of 

 tumbling streams. He does not omit quieter operations, as 

 his clever sketch of Thames meanderings shows; but his 

 favourite river theme is the mountain torrent, with its mark 

 L hi the rock which it cuts through. After mentioning many 

 instances of this sort of action, he observes that " at the 

 Devil's Bridge, near Aberystwith, a stream has sawed a groove 

 in the blue slate. It is ninety feet deep, and about six wide. 

 . . . . The rivulet has ploughed a groove at the bottom of 

 a curve ; it has turned V into Y." 



After studying the river marks, the next step is to learn 

 those of large floating ice masses and glaciers ; and for students 

 who cannot travel as extensively as Mr. Campbell has done, 

 his sketches will prove an invaluable substitute. The glacier 

 leaves its characteristic tool marks. " Wherever it goes the 

 ice tool grinds ; it works broken stones into polished 

 boulders, boulders into mud, fractured rocks into roches mouton- 

 nees, and mountain glens into rounded, polished, striated rock 

 grooves, whose ground section is a curve * — *. When the ice 

 melts, floating chips are left in the groove in their order." 



Mr. Campbell gives a good description of the various tool 

 marks left by moving masses of ice. The ice may polish rock 

 surfaces ; it may mark them with strige, or scratch them with 

 sand hues, or score them, or groove them, or make deep 

 grooves, or hollows, or glens, or in passing over rocks that 

 wear unequally, it may give them a bossy or mammillated 

 form, and thus make roches moutonnees. Yery characteristic 

 sketches of these several effects are given in the two volumes 

 of Frost and Fire. 



No geologist doubts that great masses of floating ice, such 

 as can now be studied in Polar regions, and huge glaciers, 

 of which Switzerland, Norway, and other localities afford good 

 specimens, were the kind of tools which in former ages did 

 very extensive and important work in fashioning the surface of 

 the globe. In many cases the "tool marks" are distinct 

 enough to point clearly to their origin; but when certain 



