MICROSCOPIC FEATURES 843 



laths have a prominent radial or spherulitic arrangement; the somewhat 

 coarser rock from the centers shows the same arrangement, but less prom- 

 inently. The laths are minute and not greatly twinned. In the coarser 

 varieties extinctions up to 20° are shown, and the feldspar is probably 

 andesine-labrador ite . 



In none of the slides is there any determinable pyroxene. In the finer- 

 grained rock it is quite certain that no pyroxene ever crystallized out; 

 in the coarser rock a trifle may have done so, as there are small, scattered 

 patches of calcite and other alteration products between the feldspars, 

 which may have resulted from the alteration of small pyroxenes. But 

 this is not thought probable, and, in any event, the amount is small. 



In all of the slides there are occasional altered areas which have the 

 polygonal outlines of porphyritic crystals. Many of these have the shapes 

 and angles of olivine, and all are probably original crystals of this min- 

 eral. The usual alteration is to a chlorite aggregate, often with traces 

 of the mesh structure of serpentine ; but sometimes the mineral is re- 

 placed by crystalline calcite. 



Much opaque material is seen in all the slides. There is a little pyrite 

 in small but determinable crystals. There is much finely divided, black 

 matter, which may be magnetite, though none of it shows the character- 

 istic luster of that mineral, being perhaps too minute. But the most 

 characteristic opaque mineral in the slides presents optical characters 

 new to our experience. The chemical analysis suggests that it is graphite, 

 or some graphitoid mineral, and such it most probably is; but its luster 

 is entirely different from that of ordinary graphite. It shows instead in 

 reflected light an even, light gray sheen, quite unlike the appearance 

 which metallic lustered minerals show. 



All the slides contain much calcite. It fills the amygdules, solidly 

 welds up the host of cracks which ramify everywhere through the lava, 

 and also occurs in irregular patches through the lava. In some of the 

 amygdules a little quartz accompanies it. At a rough estimate, it forms 

 between 5 per cent and 10 per cent of the mass of the knob. It gives 

 eloquent testimony to the shearing which the knob has experienced. The 

 calcite filling cracks shows everywhere prominent u inhibitory extinction 

 and polysynthetic twinning, such as are characteristically developed in 

 this mineral under compressive stress. 



The limestone inclusions in the balls have had a slight chilling effect 

 upon the lava, producing a border of clear, green glass, packed with 

 finely divided, black, opaque matter. A slight amounl of corrosion has 

 occurred, the limestone border being etched away, fragments separated 

 and inclosed in lava and in whole or in pari dissolved. The border of 

 the limestone, against the lava, is more coarsely crystalline than the re- 



