FIELD GEOLOGY. 



according to their age, although not to so great an extent, 

 and from causes that are not so well understood. 



But in consequence of the changes which rocks have 

 undergone subsequent to their formation, it is not at all 

 times easy to decide off-hand even to what class a speci- 

 men belongs it must then be subjected to certain tests 

 for its determination. There are simple tests for appli- 

 cation in the field to ascertain the class of rock that is 

 under examination ; and more delicate tests, involving 

 the use of apparatus at home, by which the field results 

 may be checked and extended. In many cases the sim- 

 ple directions given below will not go far enough to- 

 wards accurate solution, but it will then be found advis- 

 able to consult such works as are devoted to the subject, 

 or to obtain the assistance of a professed mineralogist 

 for the ultimate determination of a difficult specimen. 



In the many valuable works on Petrology and Miner- 

 alogy that have been published, the usual plan adopted, 

 and in such works the only one possible, has been to 

 name a rock or a mineral, and then to describe the re- 

 sults obtained by subjecting it to certain processes. But 

 to the geologist, who goes into the field and encounters an 

 unknown rock, this method of its determination involves 

 a vast amount of labour and perseverance. The object 

 aimed at in the compilation of the following Tables is 

 to save much of this labour by reversing the process,, 

 tabulating the results obtainable, and deriving therefrom 

 the kind of the rock subjected to experiment. This 

 must necessarily be done within confined limits ; but a 

 fair approximation to the desired result may be thereby 

 obtained, with indications for more detailed operation. 

 It should be unnecessary to insist upon what all text- 



