162 



I. Very coarse grained diallage from Baste. 



II. Coarse grained diallage (altered) from same locality. 



III. Diallage from Baste. Analysis by KOHLER, quoted by STRENG. 



IV. Augite from Gabbro, Baste. This mineral was surrounded by 



hornblende. 



V. Augite associated with the diallage of analysis I. 



i. a. in. 



Si0 2 ... 50-72 ... 52-13 ... 52-11 



A1 2 3 ... 3-38 ... 6-18 ... 4-49 



Fe 2 3 | j 1-14 ... 0-00 



FeO ) I 9- 06 ... 10-88 



MnO ... ... 0-14 



CaO ... 13-13 ... 14-32 ... 16-83 



MgO ... 11-54 ... 17-30 ... 14-23 



H. 2 O 1-12 0-73 0-90 



' 97-37 101-00 99-44 



Sp. Gr. 3-13 3-17 



I. Hornblende varying from bright green to dark green. 



II. Dark chesnut-brown hornblende from a gabbro poor in diallage, Baste. 



III. Hornblende associated with the diallage whose composition is given 



in analysis II. of the preceding list, 



The change of diallage to uralitic and actinolitic hornblende may also be 

 observed in some of the Lizard gabbros. In this case the original mineral is 

 replaced by an aggregate of hornblende fibres which lie approximately parallel 

 to each other in the central portions of the mass, but which run out at the 

 margin into a fringe of actinolitic needles. These needles invade the colourless 

 felspar substance with which they are surrounded, as is well shown in fig. 2, 

 plate XVIII. The augite of the dolerites exhibits the same tendency to pass 

 over into hornblende. This is well seen in the case of the Scourie dykes, in the 

 so-called greenstones of Devon and Cornwall/ 1 ' and in the black dykes of the 

 Lizard peninusula. 



As already stated, the secondary hornblende may be either brown, green or 

 colourless. The pleochroism of the green varieties is generally as follows : 

 =pale greenish yellow, /3=dark or pale yellowish green, 7= rich deep green 

 or bluish green. 



In considering the distribution of augite and hornblende in the basic 



(1) See papers by Mr. J. A. PHILLIPS on the -so-called greenstones of Cornwall, Q.J.G.S., 

 .Is. XXXI [ ami XXXIV. 



