THE MICROSCOPE IN MINERALOGY AND GEOLOGY. 91 



two axes of double refraction, or rather axes around which 

 double refraction occurs. Thus nitre crystallizes in six- 

 sided prisms, with angles of about 120. It has two axes 

 of double refraction inclined about 2J to the axes of the 

 prism, and 5 to each other, so that a piece cut from such 

 a crystal perpendicular to the axes, shows a double system 

 of rings when a ray of polarized light is transmitted. 

 When the line connecting the axes is inclined 45 to the 

 plane of polarization, a cross is seen, which gradually 

 assumes the form of two hyperbolic curves on rotating 

 the specimen If the analyzer be revolved, the black cross 

 will be replaced by white, the red rings by green, the yel- 

 low by indigo, etc. These rings have the same colors as 

 thin plates, or a system of rings round one axis. Mica 

 has two sets of rings, with the angle between the axes of 

 60 to 75. Magnesian mica gives an angle of 5 to 20. 

 Determination of the Origin of Rock Specimens. Mr. 

 Forbes has shown that the primary or eruptive rocks, 

 consisting chiefly of crystallized silicates, with small 

 quantities of other minerals, are developed as more or 

 less perfect crystals at all angles to one another, indicat- 

 ing the fluid state of the mass at some previous time. 

 The secondary or sedimentary rocks consist of rocks 

 formed by the immediate products of the breaking up of 

 eruptive rocks, or are built of the debris of previous erup- 

 tive or sedimentary rocks, or composed of extracts from 

 aqueous solution by crystallization, precipitation, or the 

 action of organic life. The accompanying figures, selected 

 from Mr. Forbes's article in the Popular Science Review, 

 well illustrate this method of investigation. Plate II, 

 Fig. 45, is a section of lava from Vesuvius, magnified 

 twelve diameters, showing crystals of augite in a hard 

 gray rock. Plate II, Fig. 46, is a volcanic rock from 

 Tahiti, consisting of felspar, with oliviue and magnetic 

 oxide of iron, and numerous crystals of a pyroxenic min- 

 eral. Plate II, Fig. 47, is pitchstone from a dyke in new 



