272 F. I). Adams — Melilitebearing Rock. 



very considerable alteration. The comparatively large per- 

 centage of potash in the Canadian rock is probably derived 

 principal^ from the biotite which is present in large amount, 

 while the Hochbohl basalt is nearly free from it. This bio- 

 tite which is of a rich brown color occurs in individuals which 

 although occasionally presenting a good crystalline form are 

 usually irregular in shape. The basal sections are black or 

 nearly so between crossed nicols and in convergent light show a 

 cross, the arms of which separate somewhat when the section is 

 revolved, indicating that the mineral is biaxial with a small 

 axial angle. The lateral sections show the usual eminent basal 

 cleavage with parallel extinction and a strong pleochroism, pale 

 yellow sometimes nearly colorless, to deep yellowish brown. 

 When a percussion figure is obtained and the optical orienta- 

 tion of the mica, studied it is found to be different from that of 

 ordinary biotite and like the mica in the Alno rock,* to belong 

 to Tschermak's first series, it is therefore anomite. 



Around the biotite phenocrysts a narrow zone somewhat 

 deeper in color but similarly orientated is often seen. It is 

 distinguished from the inner portion of the crystal, not only 

 by its color, but frequently also by the presence of minute 

 inclusions, for the most part little pyroxenes like those of the 

 groundmass, which are wanting in the inner portion. This 

 zone is sharply defined against the inner portion and is fre- 

 quently seen to build out the more or less rounded core into a 

 more perfect crystalline form. The outer edge, although fre- 

 quently minutely serrated, displays crystalline outlines. There 

 has evidently been, first a separation of biotite in large pheno- 

 crysts, followed by a partial re-absorption and then a second 

 crystallization of biotite, occurring partly in the form of zones 

 about the remnants of the biotites of the first generation and 

 partly as smaller individuals both in the form of phenocrysts 

 and in the ground mass. The biotite of the first generation is 

 lighter in color and free from pyroxene inclusions, while 

 these occur abundantly in that of the second generation. 



Olivine. — In the hand specimens the mineral is sometimes 

 pale green but more frequently of a beautiful bright red color. 

 It is usually well crystallized, especially the smaller pheno- 

 crysts and is traversed by a multitude of irregular cracks in 

 addition to the lines of imperfect cleavage. When examined 

 in thin sections by reflected light the color of the red olivine 

 crystals is seen to be due to the presence of irregular wavy 

 lines of ferric oxide of a brilliant red or orange color. That 

 the ferric oxide, however, is present in but small amount is 

 seen from the accompanying analysis I, of this red olivine by 



*F. Eichstaclt: "Anomit fran Alno, Vesternorrlands Ian," G-eol. Foren. i, 

 Stockholm F6rh., 1884, No. 87, 194. 



