RELATION OF OTTRELITE AND CHLORITE. 153 



tion of these minerals or an infiltration of chlorite parallel to the basal 

 cleavage of the ottrelite. Further investigation, however, showed that 

 the chlorite as often traversed the ottrelite irregularly in bifurcating veins 

 and enclosed parts of it (see figure 2). A study of the nature of these 

 veins convinced me that the chlorite is an alteration product of the ottrel- 

 ite. The edges of the veins where they traverse the ottrelite transverse 

 to the basal cleavage are jagged, the saw-like teeth projecting along the 

 composition faces or basal cleavage. The chlorite in such cases is dis- 

 tinctly made up of little fibers which have arranged themselves parallel 

 to one another and to one set of twinning lamella?. lanes of inclusion 

 once continuous in the ottrelite now stop short against interlaminated 

 areas of chlorite, showing the evident secondary nature of the latter min- 

 eral. In other places the chlorite is developed along the basal cleavage 

 of the ottrelite, leaves this cleavage and follows one of the prismatic 

 cleavages, and then again follows the basal cleavage, making one con- 

 tinuous line, thus producing steps, in much the same manner that garnet 

 commonly undergoes this alteration. 



Optical Characteristics. — Cores of unaltered ottrelite remain in the chlo- 

 rite, and the pleochroic zones, once in the parent, are seen again in the 

 secondary mineral, while the relationship of maximum pleochroism of 

 these zones to the greatest pleochroism of the chlorite is handed down 

 as well. This metasomatic phenomenon has not been observed in other 

 phases of the ottrelite-bearing conglomerate thus far studied by me. 



GROUNDMASS OF THE BOCK. 



Mineralogic Constituents. — The background of the rock is composed of 

 quartz and feldspar as principal constituents. 



Feldspar. — The feldspar is fresh and glass} 7 , untwinned, and is prob- 

 ably albite, but it is hardly abundant enough in the ottrelite bearing 

 phases to make the rock a gneiss even in mineralogic composition ; 

 and, structurally, a gneissic habit has been nowhere observed where 

 ottrelite exists to any extent. 



Sericite. — Sericite is also abundant and occurs in minute prisms be- 

 tween the interlocking quartz grains and generally inclosed by the albite 

 but rarely by the ottrelite. It also incloses lines of rutile dots arranged 

 parallel to its cleavage planes, and next to the ottrelite it is the last 

 formed mineral. 



Anatase. — Associated with rutile in the groundmass are groups of stout 

 and slender prisms and plates having a very high single and double 

 refraction and a variable color, from brownish-yellow to blue, even in the 

 same individual. These I identified as anatase. and for verification I 



