MINERALOGY, 95 



light shows no bandings, and is consequently parallel to the brachy- 

 pinnacoid, while a cleavage line indicates the direction of the base; 

 hence we know the position of the crystal section. As is usual, it is 

 plain from the figure that the most of these needles are parallel to the 

 vertical axis, since they make an angle of 1 12° with the cleavage. Parallel 

 to the base there are only a few, but those that lie in this direction are 

 all notably very long. Both of these sets of needles are in the plane of 

 the section or brachy-pinnacoid. There arc, then, two more sets, making 

 with one another a little more than a right angle, which is nearly 

 bisected by the vertical. These needles are also numerous, but they do 

 not lie in the plane of the section, but pass very obliquely through it, 

 and hence lie in the plane of some octahedral faces, the angles of which 

 cannot be ascertained on account of the obliquity. The needles are 

 referred to augite by Schrauf. These structural directions indicate an 

 interior development of these irregular grains, according to quite a com- 

 plex crystalline form. 



The labradorite is often crystallized in the diabases, giving to them a 

 pretty porphyritic character. These crystals are often quite noticeable, 

 both from their form and the beauty of their sections. The feldspar 

 of a somewhat decomposed gabbro from Waterville was analyzed by Mr. 

 E. S. Dana,* with the following result : 



Silica, 52.25 



Alumina, ............ 27.51 



Iron sesquioxide, .... 1.08 



Lime, 13.22 



Magnesia, ........... .99 



Soda, 3.68 



Potash, 2.18 



100.91 



The accession of potash and all the other characteristics of change are 

 here seen, but only in an incipient form. A point of interest is remarked 

 by Mr. Dana, that the New Hampshire labradorites are remarkable for 

 their high percentages of lime. 



* Am. y. Sc!., iii, vol. iii, p. 30. 



