236 OUTLIM > 01 1 11 I.D-GEOLCX.N IAKI n 



and petrographical grade of rocks which he cannot satis 

 ily name by any of the- usual methods available to 

 him. By far the must valuable aid in this respect is 

 supplied to him in the examination of powders or of 

 thin slices of rocks with the microscope. He may essay 



rations, though when he can 



the slides satisfactorily made for him he may save time 

 for other work, and have the slides better made. 



Pulverisation. Much may be learnt regarding the 

 mineral constituents of rocks by examining their 

 powders. For this purpose a small fresh fragment of 

 the rock to be studied is placed in a steel mortar and 

 reduced to powder, but without any grinding motion 

 that would break down the particles into dust. The 

 powder thus obtained should be sifted on fine muslin. 

 A little of it is then washed in a solution containing 

 two or three drops of strong gum in an ounce of water, 

 to remove the finer dust and leave the clean rock- 

 particles that are to be examined. These particles 

 are then lifted upon a clean glass-slide and carefully isol- 

 ated from each other, in order that each may be looked 

 at separately. When the slide is heated the grains will 

 be found to adhere to the glass. They may then be 

 studied with the microscope, or if the slide is intended 

 for preservation and future use, it should be gone over 

 carefully under the microscope to see if the particles are 

 sufficiently illustrative, and to reject what is useless from 

 thickness or size. A little Canada balsam should be 

 put over the particles, and the slide should then be 

 heated and finally protected with a thin cover glass as 

 explained on p. 244. 



