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160 



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GEOLOGY. 



[Sect. VI. 



H 



Tlieoretical 



from discussing many of the questions which ought esi)e- 

 cially to engage the attention of a sea voyager. As he 

 will probably visit many volcanic regions, Dr. Daiibeny's 

 ' Treatise on Volcanos' would be extremely useful ; and 

 a list is there given of special treatises on the volcanic 

 countries likely to be visited hy him. The ' Description 

 Physique des Isles Canaries,' by Von Buch, may be cited 

 as a model of descriptive powers. The voyager in the 

 Temperate and Polar regions ought to have Agassiz' 

 work on Glaciers. 



The geologist fortunately requires but little apparatus ; 

 a heavy hammer, with its two ends wedge-formed and 

 truncated ; a light hammer for trimming specimens ; some 

 chisels and a pickaxe for fossils ; a pocket-lens with three 

 glasses (to be incessantly used) ; a compass and a clino- 

 meter, compose his essential tools. One of the simplest 

 clinometers is that constructed by the Rev. Prof Henslow : 

 it consists of a compass and spirit-level, fitted in a miall 

 square box ; in the lid there is a brass plate, gradu- 

 ated in a quadrant of 90 degrees, with a little plumb-line 

 to be suspended from a milled head at the apex of the 

 quadrant. The line of intersection of the edge of the 



clinometer, when held horizontally, with the plane of the 

 stratum, mv 



V_/ 



M>5 lf 



^ -■* 



y^ 



strike, range, or direction ; and its dip 



or inclination, taken ... right angles to the strike, can be 

 m.easured by the plumb-line. In an uneven countrv, 

 It is not easy without the clinometer to judge which is 

 the iu... of greatest inclination of a stratum ; and it is 

 always more satisfactory to be certain of the angle than 



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