FENNER, THE WATCHVNG BASALT 125 



The appearance is so unlike that of the commoner form that numerous 

 tests were made to establish its identity. Some of these were as follows : 



1. All sections giving maximum interfereuce (perpendicular to optic nor- 

 mal) give practically parallel extinction. 



2. Sections perpendicular to both albite twinning and to basal cleavage give 

 extinction angles of 13° to 16°. 



3. Crystals which were found to be nearly or quite perpendicular to the 

 acute bisectrix Z give extinction angles of 11° to 18° with the basal cleavage. 



4. In most crystals, a combination of albite and Carlsbad twinning is found. 

 By selecting those crystals which give approximately maximum extinction for 

 albite lamellae in both halves of the Carlsbad twin, corresponding angles of ex- 

 tinction were found as follows: 121/4° and 131/^°, 121^° and 131/2°, 13i^° and 

 141/2°, 111/2° and 131/2°. 



5. The index of refraction is less than that of balsam. 



With all these facts in mutual accord, it can hardly be doubted that the 

 mineral is nearly or quite pure albite. 



The crystals are many times the size of the form of albite first de- 

 scribed, attaining a maximum length of 3 mm. or more, being thus com- 

 parable with the crystals in granitoid rocks. Carlsbad twinning is very 

 common. The two parts are joined along a somewhat irregular line, as 

 shown in fig. 4. The most characteristic feature is the curiously mottled 

 appearance, due to the irregular manner in which the albite twinning 

 lamellas are placed in Juxtaposition. The lamellse do not in general 

 traverse the whole length of the crystal, but often they stop abruptly. 

 Between crossed nicols, the different interference colors thus brought to 

 view in a single crystal have a patchwork effect, and careful study shows 

 that, curiously enough, the form of the patches outlined is suggestive of a 

 multitude of lathlike plagioclase crystals disposed more or less at random. 

 It is conceived as possible that this is a clue to the manner in which these 

 large crystals of albite originated ; that in fact each individual is built up 

 of a multitude of labradorite laths of the original basalt, which have been 

 incorporated within a single expanding crystal, and whose molecules have 

 been swung into approximately correct crystallographic orientation. That 

 the crystallizing forces of the major crystal were not able to exert rigid 

 control throughout is indicated by certain anomalies in extinction and 

 by a curving of the ends of the crystals which is frequently shown. 



In slide 129, several groups of albite crystals of the large type shown 

 in slides 133 and 134 appear in the midst of apophyllite. The borders 

 are corroded and the interiors muddy, and they are plainly remnants 

 giving way to the apophyllite. It is not improbable that a great deal of 

 secondary albite of this form appeared among the first products of altera- 



