113. These methods (both 2 and 3) are often not applicable on account of the tendency of the crystals in an effusive rock to parallel orientation, which may be so marked that the rock section does not show any favorable sections of the plagioclase.

114. These extinction angles, as well as those previously given, are those of only a few type feldspars of definite composition. As the composition varies through a long series, so the extinction angle changes, one being a function of the other.

For a complete list of compositions and related extinction angles, see Iddings’ Rock Minerals and Lévy & Lacroix’s Les Minéraux des Roches.

115. Iddings’ Rock Minerals, p. 228, Wiley & Sons, 1911. Étude sur la Détermination des Feldspaths (troisième fascicule), Michel Lévy, Paris, 1904.

116. In quartz ω is the refractive index of the ray with vibration direction ∥ a [that is the direction of vibration of the faster ray (the ordinary ray)]. Hence ω is direction ∥ a and ε ∥ c. In the feldspars; α ∥ a, γ ∥ c and β ∥ b.

117. Am. Jour. Sci., May, 1906. This method is specially useful in detecting presence of orthoclase, when plagioclase is the dominant feldspar.

118. These tests are only possible on pure and fresh material. The specific gravity increases with the Ca % (albite 2.62, anorthite 2.75).

119. In clear unstriated granules, which may be distinguished from quartz by biaxial interference figure in convergent light.

120. The tendency of labradorite in gabbros to twinning, after both Albite and Pericline laws, is to be noted.

121. Werveke’s (N. J. B., 1883, II, 97) theory is that a twin lamination may be caused by the forces producing mechanical deformations, as movement in the magma and mountain making pressure. Such lamellæ are characterized by the fact that their extent and course seem to depend on fracture lines in the crystal.

122. The derivation from pyroxene and amphibole appears to be doubtful, see Weinschenk’s Gesteinsbildenden Mineralien, 1901, p. 121.

123. Harker’s Petrology for Students, p. 63, 1895.

124. Sections can be obtained from Voight & Hochgesang, Göttingen; C. Marchand, rue Censier, 16ter, Paris; W. H. Tomlinson, Swarthmore, Pa., and G. D. Julien, 3 Webster Terrace, New Rochelle, N. Y.

125. Made by G. D. Julien, 3 Webster Terrace, New Rochelle, N. Y. Price: Power lathes (complete), $150.00 up; Foot-treadle lathes, $90.00 up.

126. A Crocker-Wheeler motor of ¼ to ½ H. P. will be large enough for an ordinary laboratory machine.

127. Made by Voigt and Hochgesang, Göttingen. Price, $15.00.

128. Saws charged with diamond dust can be obtained from Elisha T. Jenks, Middleboro, Mass. Price in 1910, $1.35 per diametrical inch. Foreign saws 6″ and 8″ diam. can be bought from G. D. Julien, New Rochelle, N. Y.

129. School of Mines Quarterly, Vol. XI, p. 32.

130. “Half a pound or less of ordinary shellac is melted in a flat-bottomed open vessel over a Bunsen burner. Then an equal quantity of Venice turpentine is carefully added under constant stirring. The mass should be allowed to boil for about ten minutes, during which the stirring is continued. Then small quantities are poured in separate heaps on an iron plate or other cold surface and rolled into sticks about seven inches long and half an inch thick.”

131. Square or standard 26 × 46 mm. glass slides are recommended instead of the oblong slides (1 × 3 inches), which are very apt to project beyond the edges of the stage and be struck by the fingers while rotating the stage.

132. Reference can be made to the publications on this subject by: Borichy, Behrens (Behren’s translation by Judd), Haushofer, Huysse, Klément, Streng, Rénard, etc.

133. In case an acid is to be used which would attack glass, the cover-glass can be replaced by a thin perforated disk of platinum.

134. The bases in solution can be determined by different methods of analysis, for which the student is referred to more elaborate works on this subject. In some cases it may be very advantageous to treat the section with acid, to remove certain soluble constituents, when other minerals not distinctly seen at first may be made more apparent.

135. The gas may be H₂S from a soluble sulphide, in which case the solution containing the bubbles will color filter paper moistened with lead water.

136. Geol. and Nat. History Survey of Minn., XIX., Ann. Rept., p. 42; also A. J. Moses, Characters of Crystals, p. 147.

137. The symmetry of the etched figures would, of course, be related to the system of crystallization.

138. A. J. Moses, Characters of Crystals, Chap. XVI.

139. When it is desired to preserve the section and at the same time to study the surface covered with a film of air, the edges alone of the cover-glass should be cemented.

140. Geol. and Nat. Hist. Survey of Minn., XIX, Ann. Rept., p. 50. Thin sections of 2–3 sq. mm. area are of a convenient size, and they should be subjected to a red heat for (1½)-3 minutes. Too long a continuance of heat may render the sections too dark or lessen their transparency or produce melting.

141. This test may vary, in many cases graphite not being consumed even after long heating.

142. The isolation of material for investigation is of more interest for the lithologist or chemist than for the student of optical mineralogy; therefore only a very brief outline of some of the methods employed will be given.

143. The grains passing through different meshes are investigated microscopically to ascertain which size grains are homogeneous; the rest of the sample should then be reduced to grains of this size.

144. Further reference can be made to Iddings’ Rock Minerals, pp. 25 and 95, 1911. A convenient form of apparatus is also described in School of Mines Quarterly, Vol. X., p. 284, 1889.

145. S. L. Penfield, Am. Jour. Sci., Vol. L., p. 446, 1895. In this article a convenient form of separating apparatus is also described.

146. A convenient list of minerals, arranged in the order in which they would be attracted by increasing the force of the electro-magnet, is given in Weinschenk’s Tabellen.

147. For micro-chemical work the reagents should be applied in very small drops, which spread out on glass to discs 2 mm. in diameter. For manipulating the reagents use platinum wires, 0.5 mm. in thickness.

148. Elemente einer neuen chemisch-mikroskopischen mineral- und Gesteins-analyse, Pragg, 1877.

Translation of above by Winchell in Geol. and Nat. Hist. Survey of Minn., Vol. XIX., Ann. Report, 1890.

149. The strength of the solution should be about 3½%; for if too weak many minerals do not give satisfactory results, and if too strong a very large number of fluosilicate crystals are formed together with the separation of much silica, thus making it impossible to carefully differentiate the crystals with a microscope.

150. As most of the rock-forming minerals that would be investigated are silicates, the hydrofluosilicic solutions can also be obtained by treatment with HFl.

151. After Lévy and Lacroix.

152. Naturkunde, Amsterdam, 2, Vol. XVII., 1881. Chemical News, Vol. LXIII., No. 1647, June 1891, et seq.

Micro-chemical Analysis, Behrens (Judd), Macmillan & Co., London, 1894.

153. After Lévy and Lacroix.

154. After Lévy and Lacroix.

155. Compiled from Weinschenk’s Tables, as revised in Petrographic Methods by Weinschenk-Clark, 1912.

156. Pirsson and Robinson, Am. Jour. Sci., iv, Vol. X, Oct., 1900.

157. Harker, Petrology for Students, p. 28, 1895.

158. Have the convergent lens or condenser lowered and the analyzer out during this test.

159. The minerals are grouped according to the maximum interference colors given by sections of the thickness of 0.03 mm.

160. In the case of very small crystals, the centering must be accurate and a cover with a small hole in the center, should be placed over the top of the tube. In this way the eye is brought directly over the axis of the microscope and figures can be observed from very minute crystals.