Lawson] 



Orbicular Gabbro. 



393 



an orbule but at right angles to each other and finding the 

 same rodlike character in both sections. 



The refractive power of the mineral is very high as shown by 

 its strong relief and the large amount of total reflection of light 

 on the cracks. The thickness of the slides could be fairly 

 closely estimated from the polarization tints of the feldspar, and 

 this being kuown the polarization tints of the mineral were fouud 

 to be much higher than those of pyroxeue for the same thick- 

 ness, and to agree closely with those of olivine. Several sections 

 were obtained showing the emergence of bisectrices, and both 

 positive and negative reactions were obtained, but in those eases 

 where the bisectrix appeared to be the acute bisectrix the reac- 

 tion was optically positive. The optic axial plane is parallel to 

 the direction of elongation. The dispersion is p <C The extinc- 

 tion is in nearly all cases parallel to the direction of elongation 

 of the rods and this direction coincides with the axis of maxi- 

 mum elasticity, a and therefore, with the crystallographic 

 axis b. In sections showing longitudinal cleavage cracks the 

 extinction is at variable angles but quite often parallel, the 

 latter case indicating sections approximately parallel to one of 

 the pinacoids. All of these characters agree with those of 

 olivine. It does not, however, appear to be affected by digestion 

 with hydrochloric or sulphuric acids, a test which is sometimes 

 given for olivine. 



The Plagidclase. — The plagioclase of the orbules appears to 

 be more basic than in the normal facies of the gabbro. Optical 

 tests show that it lies at the basic end of the plagioclase series 

 but its exact determination required other methods. Its specific 

 gravity as determined on isolated grains with a westphal balance 

 in the Thoulet solution is 2.724, and in powder the bulk of the 

 feldspar sinks very slowly in a solution having a specific gravity 

 of 2.748 with considerable amounts remaining in suspension for 

 over an hour and none floating. This test shows that the feldspar 

 is not less basic than bytownite. This determination was con- 

 firmed by a chemical analysis of the feldspar by Mr. W. T. 

 Schaller, a student in the Department of Mineralogy. The 

 material analyzed by him was a powder repeatedly purified by 

 separation by means of the Thoulet solution. The analysis was 

 made in duplicate and both results are given: 



