CHAPTER XII 



IGNEOUS ROCKS 



THE foregoing chapters have treated chiefly of the 

 structure of the stratified rocks, although the same prin- 

 ciples which guide us in dealing with them are also in 

 great part applicable to the Igneous rocks. These latter, 

 however, present some features of their own which mark 

 them off in strong contrast with the former, and which 

 the geologist can learn to distinguish only by actual 

 practice in the field. 



At the outset, the observer must be able to recognise 

 an igneous rock when he meets with it. After some 

 experience, he will, in the majority of cases, have no 

 difficulty in this discrimination, provided he has made 

 himself familiar with the characters of such rocks by 

 handling specimens of them. But the carefully selected 

 specimens of a museum or private collection do not 

 always convey a correct idea of the external character of 

 the rocks as they occur on the hill-side or ravine. It is 

 specially needful that the hammer be vigorously plied 

 during at least the earlier part of a geologist's study of 

 igneous rocks. He will find them so constantly decayed 

 at the surface, so thickly covered with a weathered crust, 



