44 



PROCEEDINGS OF THE NATIONAL MUSEUM, 



VOL. 66. 



The habit of these crystals is shown in the drawing, figure 4. The 

 faces shown as u (111) and s (111) are entirely dull and their iden- 

 tity was inferred from their zonal positions. Under the microscope 

 the powder of the diopside crystals was found to be colorless and 

 transparent. It is biaxial positive ( + ) mth 2V medium large, the 

 refractive indices being «= 1.672, /5= 1.682, 7= 1.702. The disper- 

 sion is perceptible, r>v. 



Titanite is a common constituent of the rock which surrounds the 

 miarolitic cavities and quite frequently a crystal abuts against a cav- 

 ity. No crystals of this mineral were seen 

 which were clearly later deposits in a cavity. 

 The crystals are of the usual resin-yellow color 

 and have the familiar envelope habit. They 

 are not better crystallized against the open 

 space than in the adjacent rock. 



HORNBLENDE. 



/ 



Hornblende is a widely distributed mineral 

 in the cavities although its amount varies 

 greatly. It is all an asbestiform (byssolitic) 

 variety which is usually in masses of cottony 

 snowy fibers although the color occasionally 

 varies to pale buff or light green. Most of 

 the cavities have only a minute wisp of the 

 fibers but the largest cavity seen, from which 

 the measured diopside crystal was taken, was 

 packed full of the cottony hornblende. The 

 hornblende is a " second generation" miner- 

 al in the cavities. The first wisp of fibers in 

 the smaller cavities does not seem to replace 

 anything, although where the action of the 

 solutions in the surrounding rock was more 

 extensive the diopsides are replaced by fluffy 

 Fig. 4— Diopside; PRISMATIC CRY- masscs of the hornblende fibers, and the 

 STAL from miarolitic CAVITY. jnatcrlals of all the cavit3-filling hornblende 

 may be derived from the diopside of the adjacent rock. Such fine 

 fibrous hornblende is widespread in distribution and was seen, in 

 thin sections, inclosed in calcite, in parallel growth on chlorite, and 

 inclosed in later albite crystals. 



Under the microscope the thicker bundles of the very fine fibers, 

 although very pale, are seen to have some color and pleochroism, 

 being blue-green parallel to the elongation and brownish-green across 

 the elongation. The thinner fibers are colorless and transparent 

 with positive elongation and an average maximum extinction, Zac, 



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