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Sect. VI.] 



GEOLOGY. 



161 



to estimate it. A flat piece of rock representing the 

 general slope can usually be found, and by placing a 



note-book on it, the measurement can be made very ac- 

 curately. In studying the cleavage or slaty structure of 

 rocks, accurate observations are indispensable. A mouth 



a book with instruc- 



s, and 



blow-pipe with its apparatu 

 tions for its use (Phillips's Mineralogy contains brief 

 directions), teaches a little mineralogy in a pleasant 

 manner. Besides the above instruments, a mountain 

 barometer is often very necessary : a portable level would, 

 in the case of raised sea-beaches and terraces, be useful. 

 Messrs. Adie and Son, of Edinburgh, sell a hand-level, a 

 foot in length, which is fitted with a little mirror on a 

 hinge, so that the observer, whilst looking along the level, 

 can see w^hen the bubble of air is central, and thus in- 

 stantly find his level in the surrounding district. This 

 is a very valuable instrument. Mr. K. Chambers, more- 

 over, and others have found, that an observer having 

 previously ascertained the exact height of his eye when 

 standing upright, can measure the altitude of any pomt 

 w^th surprising accuracy ; he has only to mark by the 

 level some recognizable stone or plant, and then to w^alk 

 to it, repeat the process, and keep an account how many 

 times the levelling has been repeated in ascending to 

 the point, the height of which he wishes to ascertain. 



A few cautions may be here inserted on the method 

 of collecting. Every single specimen ought to be num- 

 bered with a printed number (those which can be read 

 upside down having a stop after them) and a book kept 

 exclusively for their entry. As the value of many speci- 

 mens entirely depends on the stratum or locality whence 



1 



