'$7 



n many cases, so deeply corroded by the percolating 

 water which has decomposed their silicates, that he may 

 rience no little difficulty m procuring a tolerably fresh 



fracture from which to judge what the real character of 

 the rock may be. The weathered surface, however, often 

 helps in tins discrimination, for the decay of the more 

 decomposable minerals sometimes leaves the others more 

 easily recognisable than in the unaltered state. Should 

 the observer find himself at a loss how to name a rock, 

 let him take one or two chips in his pocket, wrapping 

 them in paper, with a label inside, to mark their proper 

 locality. When he gets back to his quarters in the 

 ng he may submit one or two minute splinters of 

 the rock to blow-pipe tests (p. 214), or if these be inap- 

 plicable, or give no satisfactory results, he may proceed, in 

 the manner described in later chapters, to reduce a frag- 

 ment of the stone to powder (p. 227), or to prepare a 

 slice of it for examination under the microscope (p. 228). 

 If even after all these trials the rock still puzzles him, he 

 had better give it some provisional name, and lose no 

 more time over its determination, but proceed with his field- 

 work, laying aside, however, some good typical specimens 

 of the doubtful rock for subsequent more careful analysis 

 cither by himself or some experienced petrographer. 



Having acquired more or less facility in detecting 

 igneous rocks by their lithological characters, the observer 

 may proceed to study ti. 'ares as rock-masses and 



the pan they have played in the architecture of the earth's 

 He will find by practice that for the purposes of 

 field-geology they are conveniently divisible into two 

 great series (i ) the Crystalline, including granite, syenite, 



