14 



Johnson and Warren — Geology of Rhode Island. 



Some idea of this texture may be obtained from the accom- 

 panying sketch. 



The feldspars are beautifully twinned after the albite and 

 carlsbad laws, rarely after the pericline. Extinction angles 

 measured in sections perpendicular to the albite lamellae after 

 the method of Michel Levy, indicated a labradorite, varying in 

 composition from AbjAUj to Ab 3 An 4 . Plates parallel to b. 010, 

 show the emergence of a bisectrix inclined to the normal. 



Fig. 2. The sketch shows the textural relations of the minerals in the 

 rhodose. The feldspar is ruled to show the position of the twinning lamel- 

 lae and the peculiar arrangement of the deorientated parts. The black rep- 

 resents the ore with inclusions of olivine and spinel (dotted). The olivine is 

 shown with inclusions of ore and feldspar. 



Extinction angles measured on basal cleavage plates gave values 

 of from 5 to 8 degrees. Homogeneous fragments of the feld- 

 spar free from inclusions sank in the Thoulet solution from 

 2-687 to 2-706, an average of 2*69. 



The chemical composition of the feldspar as calculated from 

 the rock analysis is that of a labradorite Ab 3 An 4 , a figure in 

 agreement with the composition indicated by the optical prop- 

 erties and the density. A well marked although somewhat 

 irregular zonal structure has been noted in sections cut parallel 

 to (5)010. The variation in the optical properties of the differ- 

 ent zones is, however, very slight and the variation in chemical 



