SEPARATION OF MAGNETIC GRAINS 117 



uy of the other minerals of the rock may be ad- 

 t to them, in which case they may be gently pounded 

 with water in the mortar, dried, and picked out afresh with 

 the magnet. The magnetic residue obtained represents 

 nearly the proportion of magnetic iron in the rock ; but 

 is almost always under the truth, because some of the 

 magnetic iron is in microscopic panicles, inclosed within 

 other constituents of the rock. This rough method of 

 analysis may be applied to basalt, dolcrite, and similar 

 rocks. By means of an electro magnet the minerals 

 which contain iron, but are not in the ordinary sense 

 magnetic, may be extracted. 



Important Afinsra/s in Rocks. To assist the learner in 

 his field-work the following list is given. It contains the 

 more important minerals which occur as essential or 

 accidental constituents of rock, and indicates briefly 

 the conditions under which each may be expected to be 

 found But for the chemical and microscopic characters 

 of these minerals, he must refer to a text -book of 

 mineralogy, or to one of the works referred to in the 

 footnote on p. 250. 



