lis U.S. Washington — Basalts of KuJa. 



The characteristic feature of this alteration is a reddish 

 brown mineral in small " club-shaped " crystals, not scattered 

 pell-mell through the accompanying augite and opacite grains, 

 but forming a sort of network, the crystals lying at angles of about 

 60° with each other, and two sets also at the same angle with 

 the prismatic edges of the hornblende crystal. At the end of 

 the hornblende crystal the small brown bodies are generally 

 arranged in slightly diverging fan-shaped tufts, spreading 

 inwards. Definite crystallographic planes cannot be made out, 

 though the blunt pointing of the terminations indicates a dome. 

 The pleochroism is very strong, vibrations parallel to the long 

 axis olive green (the least absorption), and at right angles to 

 this respectively light brown, and very dark red brown (the 

 greatest absorption). The extinction noted was, in every case, 

 parallel to the Jong axis- — in this point alone my observations 

 differing from those previously made, other writers noting 

 extinction angles of from "parallel or little inclined* (Palma) 

 to maxima of 20°f (Kilimandjaro) and 25°;}; (Syria)." It must 

 be remarked that in the Kulaites the opportunities for observa- 

 tion seem to have been far superior to any others, the crystals 

 being of fair size, quite distinct from each other, and, the 

 slides being well, made, of fairly light color. Filling the inter- 

 stices between the brown crystals are grains of colorless augite 

 and opacite. 



This form of alteration, like the other, begins at the surface 

 of the hornblende, or along cracks, and thence spreads in- 

 wards. It seems to show a preference for the dark variety of 

 brown hornblende (the green never being so altered), and fre- 

 quently the dark core is altered while the outer light zone is 

 unchanged, — apparently having been deposited after alteration 

 took place in the core. A striking fact in connection with 

 this, as with the preceding, form of alteration is the perfect 

 manner in which the hornblende crystal has, as a rule, pre- 

 served its form, while undergoing such a profound change of 

 substance. This will be referred to later. 



It has already been said that these two forms of alteration 

 seem to be separate stages of one general process. It is cer- 

 tainly true that the simple augite opacitic type follows on the 

 other in many cases, and numerous examples were noted 

 where are to be seen a core of unchanged hornblende, then a 

 zone*of the " brown mineral aggregate," and last on the out- 

 side a simple augite-opacite border. Or again, the central core 

 of hornblende has entirely disappeared, giving place to the 



* van Werveke, Neues Jahrb., 1879, 825. 

 f Hyland. Min. pet. Mitth., x, 1888, 238. 

 % "Doss. Min. pet. Mitth., vii, 1886, 515. 



