

7} 



breathed upon. Crystalline felspar rocks when breathed 

 upon often yield this iroelL Some rocks, especially 

 limestones containing animal matter or decomposing 

 iron-sulphides, yield a fetid or rotten-egg odour when 

 freshly broken. 



6. FeeL A few rocks are characterised by a 

 peculiar feeling to the to\ :ly shown 



.c hydrous magnesia n silicates, talc, chlorite, ser- 

 pentine, etc (also by some micaceous schists), which 

 have a greasy or soapy feel. In large tracts of country 

 formed of hlorite- schist, margarodite- schist, or ser- 

 in mine rock, the stones have everywhere this character- 

 istic The term '* trachyte* was originally applied to 

 certain volcanic rocks distinguished by the harsh, prickly 

 feeling experienced when the finger is passed over their sur 

 lace. A rock like chalk is said to be mtagrt to the touch. 



7. Behaviour in Haas. There are some re- 

 markably characteristic aspects of rocks which cannot 

 be judged of in hand specimens, any more than the 



ecture of a building can be told from the nature 

 of the stone employed in its construct ion. It is as parts 

 of the architecture of the earth's crust that rocks present 

 many of their most typical and individual features. 

 These broader and larger characters show themselves in 

 the outline of every hill and mountain. As illustrations 

 we may take the two contrasted groups of the stratified 

 fragmental and amorphous crystalline rocks. Even from 

 a distance the difference between these rocks makes 

 itself felt in the striking distinctions so often visible in 

 the form of mountains. Thus in \\. i . it \\i\\ be 

 noticed that two prominent sets of lines can be traced all 



