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. 1 on Pl. 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 Pl. 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 1: 2.2: 4.6. This, which is a wide variation from labradorite, is explained 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., 1x, 996. 
