PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 203 



Diabase.] 



the axial plane, and hence in coincidence with one of the bisectrices lying in the sec- 

 tion examined. This interposition causes the colors of the interference figure to rise 

 from rose-red to blue. The same result is obtained by selecting a point in the grain 

 being examined, where, by a local thinning of the section, as by an edge, the colors 

 appear in parallel bands following each other from the thin edge to the thicker part 

 in the order yelloir. m/. l>ltic. i/rccti to yellow again. By this alternation the great 

 thickness of the section is shown, and these four grand divisions of the scale of 

 Newton each correspond to the same amount of change as is produced by the quarter 

 undulation mica plate. On inserting this plate over such an edge, between crossed 

 nicols in convergent light, in the manner above stated, each of these bands rises in 

 the color scale of Newton a quarter wave-length, showing that the bisectrix in agree- 

 ment with the axis of the mica plate is the same as that of the mica plate, viz., n t . 

 Hence the bisectrix, which is perpendicular in the thin section, is or a. This also 

 shows the orientation of the section is perpendicular to the plane of the optic axes 

 and nearly perpendicular to the axis of greatest elasticity. 



It is evident that with this orientation it is not possible to distinguish, by optical 

 characters, a monoclinic crystal (augite) from an orthorhombic one (enstatite), since 

 the crystalline characters and the angles would be symmetrical with the extinction 

 in both. This is the case with all sections in the zone of symmetry of a monoclinic 

 mineral. 



A section of enstatite or hypersthene perpendicular to would show a parting 

 or cleavage parallel to 010. 



A section of augite perpendicular to might show two systems of cleavage at 

 right angles, viz., one parallel to 100 or to 001, and an interrupted cleavage parallel to 

 010, as well as the rhombic intersections of the prismatic cleavages. 



Considering the direction of elongation of the fibres, it is shown above that in 

 t enstatite they are parallel to the vertical axis lying in 010, and hence perpen- 

 dicular to the axis ,. In augite they are elongated perpendicular to the edge 

 100:010; and as they lie in 100 they are perpendicular to 010, and hence parallel to ,. 



As seen by the above figure, this shows that the mineral in this respect agrees 

 with augite. 



Again, by making use of the difference of double refraction, the same result is 

 reached in the following manner. Taking the feldspar in the thin section for labra- 

 dorite and finding its highest interference color, it proves to be bluish green. This 

 color is produced by the difference of the refractive indices of the axes of elasticity 

 that lie in the thin sectoin, /. c., by n f n p . This value, as given by the table in 

 Mineraux des Roches, page 323, is 0.008. In order to get this color for labradorite it 

 appears that the thickness oHhe thin section must exceed the range of the colored 



