202 SMITHSONIAN MISCELLANEOUS COLLECTIONS [vOL. 45 



and albite laws. The extinctions, according to the statistical 

 method, range as high as 31.5°, which would place the feldspar in 

 the class intermediate between those of the formulae Ab^An^ and 

 AbgAn^. The composition may then be represented by the formula 

 Ab^Aug and it would be classed among the labradorites. 



The remaining constituents are serpentine, calcite, and apatite. 

 Serpentine occurs as an alteration product of the olivine, appearing 

 in cracks with a fibrous structure, with fibers normal to the altering 

 surface. It shows aggregate polarization tints of dull gray, often 

 becoming bright yellow. Apatite occurs in automorphic elongated 

 prisms, enclosed in the olivine and feldspar. Calcite is also present. 



A partial analysis of this rock, together with a discussion of its 

 name and place in the new quantitative classification of igneous- 

 rocks^ will be found in Pt. 11, p. 211, of this article. 



UJARARTORSUAK 



Westward from Kaersut, the Cretaceous rocks are seen resting 

 unconformably upon a bluish-green, highly altered basalt. A little 

 to the west, the beds exposed in the sea-cliffs are dislocated for 

 several hundred feet by a fault, directly beyond which the strata 

 are cut by three dikes, the two westerly of which are parallel with 

 the bedding in places. These intrusive basalts are believed to be of 

 Tertiary age. The dikes are apparently of the same rock as the 

 intrusive sheets themselves and probably represent the vents through 

 which the sheeted material has reached the surface. Both olivine 

 and non-olivine varieties are represented in the Museum collection, 

 and from the similarity in composition and general features, the 

 detailed description of but one will be attempted, that from an inter- 

 bedded conformable sheet at Slibstenfjeld, directly back of Ujarar- 

 torsuak. 



Basalt. — This basalt (Cat. No. 75,489) is greenish-gray in color 

 and breaks with a roughly conchoidal fracture. In the greenish- 

 gray fine-grained ground mass occasional specks of yellowish-red 

 iron oxide are evident to the naked eye, as also are much larger auto- 

 morphic crystals of a light-colored and well-cleaved mineral, pre- 

 sumably feldspar. Examined with the hand lens, though not salient, 

 it is very abundant in lath-shaped forms. The rock presents a de- 

 composed appearance, and the acid test reveals the presence of 

 much calcium carbonate. 



In thin section the rock appears holocrystalline, xenomorphic with 

 lath-shaped feldspars, pyroxene, magnetite, hydrous iron ore, calcite, 



^ Quantitative ClassiUcation of Igneous Rocks, by Cross, Iddings, Pirsson, 

 and Washington. Chicago, 1903. 



