BY D. MAWSON. 461 



Class ii., dosolane. 



Order 4, quardofelic, Austrare. 



Rang 3, alkalicalcic, Tonalase. 



Subrang 3, sodipotassic, 



Hornhlende-}?anidiomorpho-hypoJiyali-to7ialase. 



Addend a. — (a) Around the margin of this intrusion, and 

 occupying numerous narrow dykes in the vicinity, is a hornblende 

 pyroxene andesite^ differing considerably in microscopic characters 

 from the hornblende andesite (just described) within; there is no 

 line of demarcation between them, the one gradually passing 

 over into the other. In addition to the greater abundance of 

 glassy base the following special characters of the minerals are 

 to be noted : — The felspars are much less perfectly developed 

 here, having neither the sharply defined boundaries nor attaining 

 to such dimensions as their fellows of the interior. The hornblende 

 gradually becomes more basic towards the margin, where it is a 

 true basaltic hornblende of a clove-brown colour and deep 

 pleochroism; this change in composition is closely followed b}^ 

 increasing breadth of opaque border. The pyroxene of the same 

 general character as before, though in larger crystals, is now 

 almost more abundant than the hornblende constituent. 



(b) A pebble from the beach at Wuss proved to be a hornblende 

 andesite nearly related to (a) above. Specially abundant in it are 

 large basaltic hornblendes^ with nearly straight extinction and 

 strong pleoohroism — 



a = light yellow. 

 b == brownish-red. 

 C = deep red (nearly opaque). 

 a < < b < r. 



(c) In a hornblende pyroxene andesite found as a pebble in the 

 river near Bartaleppe active corrosion of the hornblende 

 constituent is to be noted (Plate xxii., fig. 4), resulting in the 

 production of fine grains and dust of magnetite, and a clear 

 constituent, probably pyroxene.* 



* This is the well-known alteration by magniatic resorption affecting the 

 hornblendes of basic andesite lavas. 



