92 MINERALOGY AND LITHOLOGY. 



oustide of the crystal is always colored, and no optical property can be 

 recognized, save those of an aggregate of minute needles. 



What the product of this decomposition is, is of minor interest. The 

 change has resulted in the production of a substance of a higher specific 

 gravity, the gravity of the specimen analyzed being 2.96, while that of 

 anorthite is 2.75. It is the substance called saussurite. Hunt found this 

 saussurite in some Swiss rocks to approach, in composition and gravity, 

 to zoisite. It may be that this saussurite is one mineral or more, and it 

 may vary in composition according to the degree of the change. A 

 number of other names have been given to this product by different 

 analysts. Saussurite is the best known term among lithologists, to whom 

 the subject is of the most interest. 



Anorthite is often associated with labradorite in our basic rocks. 

 When this is the case, the anorthite is usually in large crystals or grains, 

 while the labradorite crystals are very small. The reason is, that the 

 anorthite is much less fusible, and hence in rocks cooled from igneous 

 fusion, the anorthite would crystallize first, and would have an opportu- 

 nity to form larger crystals in the still plastic mass. 



58. Labradorite [(Ca, Na^) AP Si^ 0'°]. 



This is a feldspar of much importance in our state, for it is an ingre- 

 dient of the larger part of our basic rocks. Its study is therefore one 

 chiefly of lithological interest. 



On Mill mountain in Stark, very large masses of an apparently pure 

 labradorite occur. It is an aggregate of crystals, but the microscojDic 

 sections bring to light such an amount of biotite and hornblende as to 

 show that the rock is allied to the diorites. This is the most pure 

 labradorite that we find. The rock is like one which occurs at Eden- 

 ville, N. Y., and is allied to some of Hunt's norites. 



Labradorite is subject to polysynthetic twinning. In basal sections, 

 as already explained, it can be distinguished from other species, since 

 a plane of elasticity cuts the base, making an angle of from five to seven 

 degrees with the twinning plane. 



In the labradorite from Stark, the angle is five degrees to one side and 

 six to the other, making eleven degrees l^etween the point when one set 

 of laminae is dark, and when the other set becomes dark. 



