MINERALOGY. 93 
But triclinic feldspars, and labradorite in particular, are subject to 
another system of twinning. In this case the base is the common face, 
and the axis of revolution is a line lying in the base and at right angles 
to the edge between the brachy-pinnacoid and the base. The twinning is 
repeated, as in the first case. This produces striations on the brachy- 
pinnacoid, and bandings of color in sections cut parallel to that face, and 
which are parallel to the edge between that face and the base. If, now, 
a section be cut parallel to the macro-pinnacoid, between crossed Nicols, 
both these systems of twinning will be seen at once; and in the Stark 
labradorite this is often the case, as well as in all the labradorites that 
are to be spoken of. This double system of twinning is shown in Fig. 4 
on Pl. 5. Wide bands are shown which represent the laminze parallel to 
the brachy-pinnacoid, and in the laminz @ and 4 are seen cross bands 
that are parallel with the base, and which make nearly a right angle with 
the plane of the other bands (the inclination of the base on the brachy- 
pinnacoid being 93°), and hence in two consecutive bands these striae 
make a very obtuse angle with one another (174°). In feldspars that are 
more finely striated this inclination of the laminze to one another is not 
plain, and the same cross band seems to run through several lamina, 
giving a netted appearance to the crystals. 
The labradorite of the gabbros possesses peculiarities. It was first 
shown that the feldspar of the gabbros at Waterville and Mt. Washing- 
ton river is labradorite by Mr. Dana and Mr. Hunt, who analyzed that 
which is found at Waterville, while that found on Mt. Washington was 
analyzed by Mr. B. T. Blanpied, of Hanover. These analyses are as 
follows : 
Waterville (Dana*). Mt. Washington (Blanpiedf). 
Silica, . js ‘ a ‘ : 51.03 51.50 
Alumina, 5 - : : ° 26.20 25.90 
Iron sesquioxide, . 2 z a 4.96 5.00 
Lime, . 5 . ‘ ‘i - 14.16 14.29 
Soda, . 3 é 5 F - 3-44 2.95 
Potash, . é 2 : 3 ‘ -58 50 
100.37 100.14 
This feldspart is dark in color, and is covered with fine striations. 
* Am. ¥. Sc#., iii, vol. iii, p. 49. + Hitchcook's Ann. Rep. Geology N. H., 1871, p. 27. 
{This same feldspar was also analyzed by Dr. Hunt, with a result essentially the same. I have selected 
Dana’s analysis because it was first published. Hunt obtained SiO2 50.30, Al203 23.10, Fe203 4.23, MgO 2 95 
+95» 
