I'Y i;ox i;ne-andesite. 



85^ 



those just described for labradorite, but whicli ditfor j;re;itl,v in dej^ree, the extinetion 

 angles l)eing very much larger than those of hibradoritc for this zone as given by 

 MM. Fouque et L6vy, and which must upon this ground be referred to anoi thite. An 

 especially fine example of such a feldspar, in which are combined the three sorts of 

 twinning most common to ])lagioclas(^ — albitc, peridine, ami Carlsbad — is seen in thin 

 section 79. It has been made the subject of a scries of careful measurements, which 



are indicated on the accompany- 

 ing diagram (Fig. !»). It con- 

 sists of two nearly equal halves, 

 twinned after the Carlsbad law, 

 one having well-marked cleav- 

 age, which is absent from the 

 other. Each shows striations 

 due to albite twinning, which 

 give symmetrical extinction 

 angles that are not the same 

 for the two halves. Near the 

 middle of the first mentioned 

 half is a portion twinned after 

 the law of pericline, as the 

 cleavage and extinction angles 

 and position of the axes of elas- 

 ticity sliow. The section appears 

 to be in the zone perpendicular 

 to GoPA and nearly parallel to 

 the base of the second half. 

 There is also a marked zonal 

 structure and variation of ex- 

 tinction of about 10° from the 

 center outward, being greatest 

 at the center. In the left hand 

 \indical6S,the.dir6ccion of the trace oF'the plane half the symmetrical extinction 

 oF.the opcic ax.es. angles in the marginal zone 



^indwaces thepositionoF theinterference figurey&^(i^^ 30° and 33°, while the 

 Fifi. 9-carisi.a(i twin of iihif;iodasi-. cxtinctiou iu tlic Central portion 



is 40° and i^o. In. the second half the symmetrical extinction angles are 11° and 14° 

 in the marginal zone and 24° at the center. This variation is due to a change in the 

 position of the axes of elasticity, which is shown by the fact that near the margin of 

 the unstriated end of the first half the hyperbolas of the interference figure meet in 

 the center of the field, but near the center of the same pmtion they couui together on 

 the edge of the field. 



MON XX 23 



