422 W. O. HOTCHKISS 



gence are shown. The total length of the line represents the maximum 

 birefringence that any section of the mineral considered can possess. 

 For many minerals the indices are only partly known. These are 

 indicated by drawing the line representing the birefringence solid, 

 when its approximate length is known, and any of the three indices 

 is known. When an index is known, and the maximum birefringence 

 is not known, it is represented by a dashed line either to the right 

 or left or through the vertical line, according as the index is a, 7, 

 or /3, respectively. The first case is illustrated by Forsterite, 

 — /S = 1.659, ^^*i ^^6 last three by Clinohumite, —/S = 1.670, in 

 which the maximum birefringence is not known. 



Minerals do not have constant imchangeable indices. The 

 indices vary with composition and with change in physical condi- 

 tions. These variations are indicated by giving both the lowest and 

 highest sets of indices and connecting the similar indices by fine 

 diagonal lines. This scheme serves to connect and group the various 

 members of mineral series, such as the carbonates, the feldspars, 

 the pyroxences, the olivine group, etc. It also serves to indicate 

 change in sign of uniaxial minerals such as occurs in the eucolite- 

 eudyalite series. 



As far as possible the minerals are grouped in the diagram accord- 

 ing to their relations. The most important rock-making groups 

 are put together so as to facilitate comparison. Thus the carbon- 

 ates are together, and similarily the amphiboles, pyroxenes, the 

 olivine group, the mica group, the feldspars and quartz. About 

 these are the other minerals — those with low indices, such as fluorite, 

 leucite, and the zeolites at the left, and the large numbers of "acces- 

 sory and uncommon" minerals to the right and below. 



An inspection of the diagram will indicate its usefulness. The 

 space between any two heavy vertical lines representing a difference 

 in index of 0.020, or, roughly, two times the maximum birefringence 

 of quartz, will be seen to contain comparatively few minerals, and 

 usually there is a difference in the birefringence of these which serves 

 for their easy distinction. 



The diagram serves for the ready finding of any mineral correspond- 

 ing to a known index. In order to fmd the indices of a known mineral 

 a table arranged alphabetically is given. The table gives the indices 



