MINERALS OF THE SQUARE BUTTE DARK ROCK. 409 



})leochroic : s = ]>ale steel blue ; to = pale leather brown. This dusty pig- 

 ment is very ai)t to be confined to an inner core, which is surrounded by 

 a clear colorless zone. Sometimes the apatites are of a pale red-violet 

 brown and nonj)leochroic. The crystals are bounded by the unit prism 

 and several pyramids, but they were too small to determine the planes on 

 material separated by the heavy liquids. The basal ])arting is common. 

 Cases of twinning like that mentioned by Washington * were not oV)served. 

 As shown by the analysis, the mineral is present in considerable amount. 



Olivine.— This mineral presents the usual type, but is at times of a 

 very pale yellowish color in the section and then shows a faint but clearly 

 perce])tible pleochroism in tones of yellow and white. It is generally 

 quite fresh, but sometimes has borders and patches of alteration into a 

 reddish ferruginous material. 



Biotite. — The large cleavage surfaces of this mineral, made U]) of com- 

 posite individuals,jhave been described above. It is strongly pleochroic, 

 the colors varying between a verv pale brownish orange and a deep 

 umber brown. Cleavage i)late3 appear uniaxial, but in the section, where 

 very thin edges may be found, there is enough of an oi)cning to the arms 

 of the cross in convergent light to establish it as meroxene, the usual 

 variety. The twinning and inclined extinction sometimes seen in the 

 biotites of nei)helinite and theralite rocks were not observed. 



Besides this |brown variety of biotite, there is i)resent also in much 

 smaller amount a pure deep green kind, which, from its method of occur- 

 rence, we infer has been formed from the brown one. All gradations are 

 found between them, but in such cases the brown forms an inner core 

 which changes to green on the outer edges. This green kind is partic- 

 ularly to be seen around the olivines, and especially where they come 

 in contact with orthoclase. The appearance of these colorless olivines 

 surrounded by this deep green .mantle is very striking. This variety 

 shows very little change of pleochroism or absorption ; it is uniaxial, 

 and its double refraction is e(pially strong with that of the brown. It is 

 (piite irregular in outline. 



Tlie intermediate {)0siti()n that biotite, in respect to its chemical nature, 

 liolds between olivine and feldspar has been noted by Iddingsf and is 

 shown in the analysis of its formula. Thus if we consider typical biotite 

 as (HK), (MgFe), Al, Si,0,,, this separates into (^rgFe), SiO, + (HK)., O 

 -f- Al^Oj -f 2 SiO.^, thus furnishing olivine and the oxide molecules 

 necessary for orthoclase. It is possible that this intimate relation may 

 condition the appearance of secondary biotite where olivine and ortho- 

 cla.se are contiguous. 



•Jour. Geol. ChicaKo. vol. lil, 1895, p. 25. 



fOrigin iKoeoua Rooks: Bull. Pliil. Soc. Wimliiaglon, vol. .xii, 189-', pp 10.5, ICt;. 



