94 MINERALOGY AND LITHOLOGY. 



The quantivalent ratio, in Mr. Dana's analysis of the protoxides, sesqui- 

 oxides, and silica, is as i : 3 : 5.5, which indicates a trifle less silica than 

 typical labradorite, the result being a little irregular, as remarked by 

 Mr. Dana, on account of the presence of microscopic grains of a titanic 

 magnetite, which with the greatest care could not be all separated. A 

 glance at this labradorite in a thin section indicates the impossibility 

 of obtaining pure material for analysis. The analysis of Mr. Blanpied 

 agrees very closely with that of Mr. Dana, both showing a labradorite 

 rich in lime, and containing the same impurities, as is confirmed by my 

 microscopic sections. 



The microscope indicates in this feldspar a system of twinning like 

 the labradorite of our other rocks, but the regularity and parallelism of 

 the bandings are markedly absent. The feldspar is fresh and undecom- 

 posed, and the bands receive the highest color in polarized light; but the 

 individual striations are very narrow and often irregular, on account of 

 the complex nature of the grain. The appearance of the feldspar is seen 

 in Fig. I on PI. 10. Among the microscopic impurities of this feldspar 

 are olivine, biotite, and magnetite, but more interesting than these are 

 the microscopic black needles that are represented in Fig. 5 on PI. 5. 

 The presence of these crystallites is very characteristic of the feldspars 

 of gabbros ; and they have been investigated by many observers, notably 

 by Schrauf,* who studied the feldspar of gabbros from Labrador. He finds 

 little plates, the nature of which he is unable to determine, inlaid in two 

 planes parallel to the edge between the macro- and brachy-pinnacoids, 

 but which are not planes occurring on labradorite crystals. The reflec- 

 tions from these plates produce the aventurine effect of labradorite. 

 These plates are not present in our feldspar, and hence it is not aven- 

 turine ; but the needles are sometimes present in multitudes, and most 

 abundant in the centres of crystals. They are usually found inlaid par- 

 allel to the basal and prismatic cleavages ; but in our gabbro, it appears as 

 though their arrangement were quite complex. This section in polarized 



CaO 14.07, soda and potash, 2.65=99.30. (Hitchcock's Ann. Rep., 1871, p. 27.) The quantivalent ratio of this 

 analysis is as i : 2.2 : 4.6. This, which is a wide variation from labradorite, is e-xplained by Dr. Hunt by the 

 presence of biotite which he recognized, and it is true that the rock contains some biotite; but there is none of 

 the rock which, under the microscope, is not seen to contain much chrysolite as an essential ingredient, and this, 

 it seems to me from microscopic investigation, must be the main cause of the variation, since its presence would 

 account for the variation very exactly, while the biotite is but sparingly present. 

 * Wien. Akad., Ber., Ix, 996. 



