The interference colors of all minerals here recorded are those given by sections 0.03 _mm._ in thickness (very thin sections). Footnote 83: Shown by Mügge, Joly, etc., to be caused by radiations emanating from U, Th, R, etc. Iddings’ _Rock Minerals_, 1911, p. 189. Footnote 84: Iddings’ _Rock Minerals_, 1911, p. 392. Footnote 85: See page 31. Footnote 86: R. D. Irving, _Am. Jour. Sci._, June, 1883. Footnote 87: Granites of the central Alps, where the calcite crystals are intergrown with quartz. Footnote 88: The minerals Nephelite, leucite, sodalite (haüynite and noselite) and melilite are often grouped together under the name “_feldspathoides_”; on account of their relation in rocks being equivalent to that of the feldspars. Footnote 89: Oriented in conformity to the intergrowth with augite, etc., (010) and (100) are reversed. Footnote 90: For other alteration processes, see Iddings’ _Rock Minerals_, p. 380. Footnote 91: May be difficult to determine in the case of prism zone sections, showing large extinction angles. Footnote 92: See p. 88. Footnote 93: Iddings’ _Rock Minerals_, p. 456, 1911. Footnote 94: Depending on whether the axial plane is parallel or at right angles to the clino pinacoid (010) (the plane of symmetry), we have micas of the second (biotite) or first order (muscovite). Footnote 95: The distinction between zoisite α and β (essentially orthorhombic, but may be composite triclinic twins) and clinozoisite (monoclinic close to orthorhombic) depends on differences in position of plane of optic axes; axial figures shown by cleavage plates; dispersion; anomalous interference colors; etc. See Weinschenk’s _Die Gesteinbildenden Mineralien_, p. 83. 1901. Footnote 96: Test not easily made on account of the very high order interference colors, resulting from the strong double refraction. Footnote 97: See under quartz, p. 63. Footnote 98: Iddings’ _Rock Minerals_, p. 208, 1911. Footnote 99: On account of the weak double refraction the interference figures are not very sharp or well defined in thin sections. In most cases only the black hyperbolas are seen, without any colored curves. Footnote 100: By heating feldspar crystals the axial angle decreases to 0° and then increases in the plane of symmetry (at right angles to its former position). On cooling the axial angle returns to its former position if the temperature has not exceeded 500° C. If the temperature has been 600°–1000° C. for some time the axial angle will not return to its former position. This fact may give some clew as to the temperature at which the feldspar crystals formed. Footnote 101: This change to kaolin or clay in granite is called by Dolomieu “_La maladie du granit_.” Footnote 102: See p. 31. Footnote 103: Hatch’s _Introduction to the Study of Petrology_, p. 33. Footnote 104: