Weed and Pirsson — Bearjpaw Mountains, Montana. 359 



pale green, of a wide extinction angle, good cleavage, and con- 

 tains a few inclusions of glass, biotite, and iron ore. 



The biotite is of a brown, strongly pleochroic variety, which 

 passes at times into a green one. It has the character of a 

 granitic biotite and is not veryidiomorphic. Although so pro- 

 minent megascopically, it is less in amount than the pyroxene. 

 It frequently encloses iron ore and patches of iron ore and 

 biotite are found which are evidently recrystallizations after a 

 former olivine. In a few cases a little olivine remainder has 

 been found in them. These are not like the opacite resorptions 

 found in effusive rocks, but composed of rather coarse group- 

 ings of the minerals. 



The feldspar is found in broad masses which yield wide 

 plates in the section. Only in one specimen was the feldspar 

 developed in thin, flat tables on b (010) with irregular boun- 

 daries. The broad fields of it enclose all the other minerals in 

 a poikilitic manner. In its composition it is wholly anortho- 

 clase or soda-orthoclase. This is shown by the fact that sections 

 perpendicular to an obtuse (2ET>115°) positive bisectrix give 

 extinction angles in a positive sense of 9° for the Bearpaw mine 

 type and 12° 30' for the Beaver Creek occurrence from the 

 trace of the cleavage c (001). The orientation of the section 

 is given by this cleavage, and the direction of the vertical axis 

 c by lines of altered inclusions and by a parting which appears, 

 to be parallel to a prism face ; between these the angle /? was 

 measured 63° in each case. 



The feldspar, moreover, presents all the characteristics of the 

 soda-orthoclase group, which have been so frequently men- 

 tioned in these papers and shown to be dominant in the Bear- 

 paw rocks. It has the homogeneous appearance with low 

 powers, and the patchy varying appearance with high ones 

 between crossed nicols. In one or two instances plates have 

 shown an extremely fine, delicate, scarcely perceptible twinning 

 following the albite law. The feldspar is indeed similar to the 

 cryptoperthite of the Norwegian alkali rocks. 



No trace of any lime-soda feldspar has been found in these 

 shonkinites. It did not occur in the original type at Square 

 Butte and was very sparingly present in the Yogo Peak variety, 

 a noteworthy fact when the high per cent of lime is considered. 

 The presence of a little nephelite is suspected as the rock powder 

 yields a little gelatinous silica (not due to the olivine) on treat- 

 ment with very dilute acid. We have not found it in the sec- 

 tions and only a mere trace can be present. 



The structure is the typical hypidiomorphic one characteris- 

 tic of even-grained abyssal rocks. 



The chemical composition of the Beaver Creek type is 

 shown in the following analysis made for us by Dr. H. EL 

 Stokes : 



