BlasdAle.] 



Contribution to Mineralogy . 



329 



General Description. — For the detailed study of the mineral two 

 of its occurrences were selected, a boulder containing talc as the 

 associated mineral from locality A and a portion of the mass from 

 a blue hornblende rock of locality B. The long lath-shaped 

 crystals composing the greater part of the boulder were frequently 

 imperfect and distorted, but from the interior of it a few well- 

 developed crystals were obtained. These showed only two planes, 

 the prism and the clinopinacoid. The inclination of the prismatic 

 faces (1 10) and (1 10) was found to be 54 io', though the reflections 

 obtained were poor and only admitted of approximate measure- 

 ment. Nearly all the crystals showed indications of the action of 

 pressure or other distorting factors. Those on the periphery were 

 frequently bent through angles of 5 to 20 , sometimes abruptly, 

 but more often to a more curvilinear outline. Microscopic cracks, 

 commonly at right angles to the prismatic axis of the crystal, were 

 scarcely ever absent, and were often so abundant as to render even 

 the thinnest crystals more or less opaque. The color of the mineral 

 is uniformly of a light emerald green, and crystals free from the 

 above-noted feature were perfectly transparent. Inclusions were 

 in no case observed. Two sets of cleavage planes parallel to the 

 prismatic faces were strongly developed. The specific gravity is 

 3.1 16, in which respect it corresponds closely with the mineral 

 actinolite. 



Optical Properties. — The determination of the optical features 

 of the mineral is rendered difficult on account of the microscopic 

 cracks previously noted. A section cut parallel to (01 0) when 

 examined for extinction with a Bertrand ocular gave as the aver- 

 age of seven measurements, 14 34'. Des Cloizeaux, working with 

 a St. Gothard actinolite obtained the value 15 , and Levy and 

 Lacroix* obtained the same value for Zillerthal actinolite. The 

 direction of extinction which makes the smaller angle with the 

 cleavage trace in a section parallel to (010) was shown by means of 

 a quartz wedge to be the position of least elasticity. The same 

 section examined in convergent polarized light gave no figure, 

 while a section parallel to (100) showed the emergence of an optic 



* Comptes Rendu, Tome 106, p. 777. 



