368 



The only other constituent of the rock which calls for description 

 occurs in the form of small green microlites, which are generally five 

 or six times longer than broad. They may be uniformly distributed 

 through the rock or they may be aggregated in patches, in which 

 case they are often associated with iron ores. Mr. ALLPORT somewhat 

 doubtfully referred them to hornblende on account of their pleochroism. 

 The following facts, however, point decidedly to the conclusion that 

 they must be referred to segirine. They are sometimes idiomorphic in the 

 prismatic zone, and when this is the case cross-sections are bounded 

 by the traces of four prismatic and two pinacoidal faces. The angles of 

 the prismatic faces are 87 and 93 and the pinacoids truncate the acute 

 angles, they correspond to the form (100) which is the characteristic 

 pinacoid of segirine. When examined under crossed nicols the microlites 

 extinguish parallel or nearly parallel to their length (maximum angle 

 only 2 or 3), and the major axis of depolarization is always the one 

 which most nearly corresponds with the direction of elongation. In 

 both these respects the microlites differ from hornblende and augite 

 but agree with segirine. ROSENBUSCH states that the a axis of elasticity 

 in segirine makes an angle of only 4 or 5 with the vertical axis. The 

 pleochroism is not very marked, owing to the small size of the microlites, 

 but such as it is it answers to segirine. The colour for rays vibrating, 

 parallel to the length of the crystal (i.e., parallel to a) is green; that 

 for rays vibrating at right angles to the length is often yellowish. 



The rock is a typical phonolite, as Mr. ALLPORT has pointed out. 

 It was analysed by Mr. PHILLIPS, in duplicate, with the following 



result : 



I. II. 



Si0 2 ... 56-46 ... 56-40 



A1 2 3 ... 22-29 ... 22-20 



Fe a 3 ... 2-70 ... 2-61 



FeO ... -97 ... -97 



MnO ... tr. ... tr. 



CaO ... 1-47 ... 1-35 



MgO ... tr. ... tr. 



PA - tr. ... tr. 



K 2 ... 2-81 ... 2-78 



Na 2 ... 11-13 ... 11-11 



H,0 2-05 2-05 



99-88 99-42 



Sp. Gr. 2-54 



APPENDIX TO CHAPTER X. 



Melilite has already been mentioned as an accessory constituent of 

 certain nepheline and leucite rocks. STELZNER (I) has shown that in 

 certain basalts from the Swabian Alps, from Wartenberg on the Donau 



(1) N.J. Beilage Band, II., 1882, p. 369. 



