DESCRIPTION OF IlililTSTKATIONS. 



Ill order that any object in a thin section, to wliich special reference 

 is made, may be readily found again by one studying the collection, thus 

 saving the time and patience of the stndent and leaving no room for doubt 



Fiij. 2. — Orientation of Slides. 



as to the exact spot under discussion, the following method of determining 

 the locality of such an object has been employed and is i-ecommended for 

 general use: Jiark on the stage of the microscope two radii at right angles 

 and graduate them in millimeters, beginning from the center of the stage, 

 numbering them as in the figure Place tlie glass bearing the rock section 

 10 c L 1^'" 



