72 THE AMERICAN MONTHLY [April, 



Owing to the different absorption of light, in different crystallographic di- 

 'rections, the mineral section often changes color as the stage of the microscope 

 is revolved, and this may be very characteristic of a given species. 



The analyzer is so adjusted that its plane of vibration is perpendicular to 

 that of the polarizer, so that, together, they produce a total extinction of the 

 light, if no doubly refracting substance is introduced between them. On in- 

 troducing a mineral section between the two Nicol prisms, one of two phe- 

 nomena will be produced. Sections of non-double-refracting minerals, as 

 w^ell as sections cut in particular directions from some double-refracting min- 

 erals, produce no effect whatever. Nearly all sections, however, of double- 

 refracting minerals, when introduced between crossed Nicols, produce marked 

 phenomena. On revolving the microscope stage, the field becomes black 

 once for each time that the stage is turned through an angle of 90°. In a 

 complete revolution, therefore, the light is extinguished four times. In inter- 

 mediate positions the section shows more or less brilliant polarization colors, 

 which are dependent on the mineral species, as well as on the thickness of 

 the section and the direction in which it is cut. It is further noticed that 

 sections of different double-refracting minerals, which produce these phenom- 

 ena, if placed in similar initial positions, must be revolved through different 

 angles before extinction of the light is produced. These so-called extinction- 

 a7igles show the position in the crystal of the axes of elasticity^ and are as 

 characteristic of a given species as is the cleavage angle. 



By introducing a convex lens between the polarizer and the section, and 

 removing the eye-piece, the so-called interference figures are produced. These 

 may be either uniaxial or biaxial, and if the latter ma}^ have a large or small 

 optical angle. They may be positive or negative, and may disperse the light 

 according to different laws. The position of the interference figure, with 

 reference to crystallographic directions, is a matter of the greatest significance. 

 All of these phenomena to be appreciated must be carefully studied, and, 

 though their explanation is most conclusive when studied with a com- 

 prehension of the undulatory theory of light, complicated analysis is i^e- 

 quired. These explanations may be found in the text-books of Rosenbusch* 

 and Groth.f 



It is sutHcient to state that, by means of such optical tests, it is possible, 

 and in the majority of cases an easy matter, to identify the different minerals 

 when present in microscopic crystals in a rock section. 



As soon as the microscope came into general use, it was found that many 

 minerals which had been considered rare occurred widely distributed in mi- 

 croscopical crystals. Other minerals whose crystalline form could not be 

 determined by the ordinary methods gave most conclusive proof of their 

 crystal system by their optical properties. Not only did the microscope re- 

 veal the character of the individual minerals composing a rock, but those 

 minerals which were the first to form when the magma was consolidating, 

 were shown b}^ their perfect outlines. The study of massive rocks, from 

 widely separated localities, has shown that the order of crystallization has 

 been, with few exceptions, the same for all. The great leader in petrography, 

 Heinrich Rosenbusch, has expressed this order for the holocrystalline rocks 

 by four generations, viz: — (i) Ores and accessory minerals, (magnetite, 

 ilmenite, apatite, sphene, etc.) (2) The iron-magnesia minerals, (olivine, 

 augite, hornblende, and mica). (3) The feldspathic constituents (feldspar, 

 nephelene, leucite, etc.) ; and (4) Qiiartz. The order of crystallization of 

 the minerals in a magma is, therefore, one of decreasing basicity. 



The experiments which have been carried out with artificial magmas have 



*Mikroskopische Physiographie der petrographisch wichtigen Mineraliea. Stuttgaft, 1885. 

 f Physikalische Krystallographie. Leipsig, 1885. 



