28o ALBERT JOHANNSEN 



by leaving the most abundant mineral to the last and recording 

 it as the difference. As a check, the last mineral should be 

 measured and the sum of all the readings should be the same as 

 that of the field of view. 



In tracing the peripheries of the mineral grains, when they are 

 very irregular, it is not necessary to follow their outlines abso- 

 lutely; the little variations from one side to the other compensate. 

 Furthermore, hackly ends may be averaged, long needles traced 

 to half their length but twice their width, and so on, the results 

 being usually more accurate than if the actual outlines are 

 followed. Where there are a number of very small grains, as of 

 magnetite, they may be estimated and a circle described by the 

 stylus approximately equal to their sum. A little experience will 

 show what may be done in the way of short cuts. 



The number of fields of view to be measured in a rock slide 

 depends upon its uniformity of texture, the mineral distribution, 

 and the size of grain. With small grains and uniform distribution 

 of minerals, a single field may suffice. With coarser grain or 

 unequal distribution, it may be necessary to take as many fields 

 as will cover the entire thin section, or it may be advisable to use 

 several sections. If the mineral grains do not fill the field of view, 

 or if there are pore spaces, it is only necessary to deduct the read- 

 ings of these from the reading of the circle of the entire field of 

 view and reduce the sum to loo. 



The measurement of the dark minerals is easy, but the separa- 

 tion of the colorless minerals may give trouble. Before running 

 the outline of one of these, it is usually well to swing off the camera 

 lucida, insert the analyzer, and rotate the stage to various posi- 

 tions until the boundaries are clearly in mind; then, after repla- 

 cing the camera lucida, place the edge of the mineral tangent to 

 the vertical cross-hair as before and, with the nicols still crossed, 

 trace the outline. The writer uses a Leitz microscope with simul- 

 taneously rotating nicols which makes it unnecessary to disturb 

 the position of the stage; the nicols only are rotated until the 

 mineral appears in its most pronounced color. 



With crossed nicols, or when separating minerals of different 

 refractive indices by partially closing the lower diaphragm, it 



