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 PI. 5. Wide bands are shown which represent the laminse parallel to 

 the brachy-pinnacoid, and in the laminae a and b 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 laminae to one another is not 

 plain, and the same cross band seems to run through several laminae, 

 giving a netted appearance to the crystals. 



The labradorite of the gabbros possesses peculiarities. It v/as 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 51-03 51-50 



Alumina, ..... 26.20 25.90 



Ironsesquioxide, .... 4.96 5.00 



Lime, 14.16 14.29 



Soda, 3.44 2.95 



Potash, .58 .50 



100.37 100.14 



This feldsparj is dark in color, and is covered with fine striations. 



* Am. J. Set., iii, vol. iii, p. 49. f Hitchcook' s Anyi. Rep. Geology N. //., 1S71, p. 27. 



X 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 SiOs 50.30, Al^Os 25.10, Fe^Oa 4.23, MgO 2.95, 



