216 OUTLINES OF FIELD-GEOLOGY PART n 



use, may be packed into a box which will go into the 

 corner of a portmanteau. 



The annexed Scheme shows the method of procc< 

 which the observer may follow in the blow-pipe deter- 

 mination of a mineral, extending it to further reactions if 

 necessary. He will find it advantageous to write down, 

 under each head, the behaviour of the substance, before 

 he proceeds to the next operation. 



5. Magnetic Analysis. Many dark crystalline rocks 

 contain much magnetite or other magnetic mixtures of 

 iron oxides. Some idea of the relative proportions of 

 these ingredients may be formed by reducing a specimen 

 of the rock to the finest powder in an agate mortar, and 

 then weighing out so many grains of the powder. If a 

 hammer has been used in collecting the specimen, the 

 latter should be carefully washed and rubbed with a 

 brush before being reduced to powder. The magnet, 

 protected by fine tissue-paper, may then be inserted into 

 the powder, and the magnetic particles which adhere to 

 it should be dropped into a separate dish, which is easily 

 done by pulling the magnet slightly away from the paper, 

 when the iron particles at once fall off. The process 

 should be repeated until no more magnetic grains adhere 

 to the magnet. An additional proportion of iron grains 

 may, however, be obtained by grinding the powder in the 

 mortar with water, allowing it thereafter to dry thoroughly, 

 and then when it has once more been bruised in the 

 mortar, placing the paper - protected magnet upon it 

 Minute black specks will be observed adhering to the 

 paper. The magnetic grains in the separate dish should 

 be examined with a lens to see that no considerable 



