.- 4 s OUTLINES OF FIELD-GEOLOGY PART n 



seemed to be uniform black patches arc then found in 

 many cases to inclose bright brassy kernels of pyrite. 



Transmitted Light. It is, of course, with the light 

 allowed to pass through the prepared slides that most of 

 the microscopic examination of minerals and rocks is 

 performed. A little experience will show the k 

 that in viewing objects in this way he may obtain some- 

 what different results from two slices of the same rock 

 according to their relative thinness. In the thicker one 

 a certain mineral will appear perhaps brown or almost 

 black, while in the other what is evidently the same 

 mineral may be pale yellow or green, or almost colour- 

 For many purposes of the petrographer extreme 

 thinness in the slides is necessary. 



Dichroism. Some minerals show a change of colour 

 when a Nicol prism is rotated below them. Hornblende, 

 for example, exhibits a gradation from deep brown to 

 dark yellow a mineral presenting this change is said to 

 be dichroic. To ascertain the dichroism of any mineral 

 we remove the upper polarising prism and leave only 

 the lower. If as we rotate the latter directly under the 

 stage of the microscope no change of tint can be ob- 

 served, there is no dichroic mineral present, or at 

 none which shows dichroism at the angle through which 

 it has been cut But we may often detect little crystals 

 which offer a lively change of tone as the prism goes 

 round ; these are examples of dichroism. This behaviour 

 may be used to discriminate the mineral constituents of 

 rocks. Thus the two minerals hornblende and augite in 

 many respects resemble each other. They differ in their 

 cleavage angles, but augite remains passive or nearly so 

 while the lower prism is rotated : it is not dichroic, or 



