ROSCOri ION jst 



dcicrminaiion of rocks he may propound to himself the 

 following questions : i rely crystalline, 



consisting solely of crystals of different minerals 

 laced; and if so, what are these minerals? and, Is there 

 any trace of a glassy ground-mass? If there u, he may 

 regard the rock as belonging to the volcanic series, jrd. 

 Can he detect any evidence of the devitrification of what 

 must have been at one time the glassy basis of the whole 

 rock? Thi* devitrification might be shown by the ap- 

 pearance of numerous microscopic hairs, rods, bundles 

 of feather like irregular or granular aggregations. 4th, 

 In what order did crab crystallise? This may 



often be very clearly made out with the microscope, as, 

 .stance, where one mineral is partially or completely 

 inclosed within another. 5111, What is the nature of any 

 alteration which the rock may have undergone ? I n a 

 vast number of cases, the slices show abundant evidence 

 of such metamoip Ispar passing into granular 



kaolin, augite changing into various indefinite green 

 products < into serpentine, tiuniferous 



iron into leucoxcne, while secondary calcite, quartz, 

 r zeolites run in minute veins or fill up inter 

 slices of the rock. 6th, Is the rock a fragmental one ; 

 and if so, what is the nature of its component grains? 

 Is any trace of organic structure to be detected ? 



In fine, I return once more to the main purpose of 

 this book, which is to induce the reader to cui 

 geology as an out-of-door recreation, and to give him 

 a few hints for his guidance. Apart from its healthful 

 mental training as a branch of ordinary education, geo- 



