GRANITE OF MULLAGHPERG, CO. DONEGAL. 



551 



light, the mica presents an iuterference-figure which is practically 

 uniaxial. It belongs accordingly to the biotite-group. Its absorp- 

 tion is exceedingly strong, viz., a = pale yellow ; j3 and y = black. 

 The hornblende occurs either alone or associated with the mica or 

 with a small quantity of magnetic iron-ore. Its maximum 

 extinction-angle, measured to the vertical axis in the prismatic 

 zone, is 17°. The pleochroism is marked, viz. : a = pale yellow, 



Fig. 2. — A twinned Crystal of Hornblende, showing the extinction- 

 angles of the two halves, measured to the trace of the plane of 

 twinning (oo P co). 



/3=deep brownish green, y— deep olive-green. Twinned crystals 

 occur occasionally, the twinning-plane being oo P oo (100). Fig. 2 

 shows such a twin. Traces of a cleavage parallel to a positive 

 orthodome will also be observed in this figure. A cleavage parallel to 

 the face P oo (101) has been observed in hornblende by C. Whitman 

 Cross *. 



Produced probably by the hydration of the mica, are scales of 

 a bright green chlorite. Sphene occurs in large but isolated grains 

 enclosing granules of magnetite and, in one case, a well-contoured 

 crystal of oligoclase. Occurring as a secondary product in the altered 

 felspar are granules of epidote. 



Of all these minerals the oligoclase shows perhaps the greatest 

 tendency to the assumption of crystalline contours. It is, however, 

 nearly equalled in this respect by the hornblende. This, combined 

 with the fact of its occurrence as an inclusion in sphene, should 



p. 386. 



Studien iiber bretouische Gesteine," Min. und Petr. Mitth. iii. (1881), 



