444 K S. Dana — Petrography of the Sandwich Islands. 



beyond description. Groups of these forms radiating from a 

 center are common.* 



The accompanying figure, 1, shows several of the more 

 complex of these forms (a, b from this specimen) and gives 

 a fair representation of this remarkable structure. Figure 2 

 gives the appearance of the entire field of the microscope, 

 showing forms like the frost crystals occasionally seen on 

 a stone pavement ; this figure is simplified by the omission of 

 some of the less defined parts. 



Some of the simpler rosettes are made up of both feldspar 

 and augite alike radiating from a common center ; and fre- 

 quently the extremities of the feather ends are feldspar indi- 

 viduals. Figure 3 gives a detailed drawing of part of one of 



Detailed drawing showing the 

 feather-like groining of augite and 



Feather-augite in basalt from Mokuaweoweo. felds P ar - Ma S Qified 10 ° times - 

 Magnified 60 times. 



the groups. It would seem that the feldspar was as usual first 

 separated, and the augite as it crystallized out into these den- 

 dritic forms drew the feldspar needles into position with it. 

 The two minerals are sometimes so intricately involved with 

 each other that it requires close examination to separate them. 

 In polarized light the distinction comes out more sharply. 



* Mr. H. Hensoldt of New York has caUed the writer's attention to an augitic 

 lava from Tahiti in which the pinkish, pleochroic augite is present in radiating 

 groups of acicular crystals, often having a nucleus of chrysolite. The section is 

 one of very exceptional beauty and interest, although the arrangement of the 

 augite is hardly to be compared with that here described, since the individual 

 crystals are sharp and geometrically grouped — after the manner of the tourmaline 

 in luxullianite — which is in marked contrast to the feather forms of the Mauna 

 Loa augite. 



