256 PKOE. T. Gt. BONNET ON HOENBLENDE-PICRITE 



tinous rocks, and think that probably we have here an altered ensta- 

 tite*. The slide, however, does not exhibit in any part the peculiar 

 brassy lustre of bastite. 



No. I., though in many respects agreeing with the first described, 

 has one or two varietal differences. There is undoubtedly a little 

 plagioclastic felspar, rather decomposed, with a considerable amount 

 of an almost colourless mineral, containing fine earthy granules, 

 acting rather feebly on polarized light, and without a very charac- 

 teristic cleavage, which I am disposed to regard as akin to augite. 

 Some of the brown hornblende crystals seem to pass through a 

 narrow green border into a closely cleaved colourless mineral like a 

 diallage, with a larger extinction-angle than that of hornblende. 

 Indeed, more than one specimen in the above-described rocks, though 

 strongly dichroic, more resembles diallage in aspect, and has an 

 extinction- angle too large for hornblende. The Schriesheim rock, 

 said to contain diallage, gives a similar discrepancy. I am disposed 

 to explain the apparently contradictory results afforded by these 

 rocks by supposing that the mineral originally predominating in the 

 picrite was a pyroxene, and that we find it now in various stages 

 of conversion into hornblende. 



I am indebted to the kindness of my friend Mr. J. A. Phillips, 

 F.K.S., for a duplicate analysis of the rock collected on the road to 

 Ty Croes station (No. IV.), which I subjoin : — 



I. II. 



w . f hygrometric 0-60 0-53 



Water \ combined 2-84 2-90 



Silica 42-94 42-79 



Alumina 10-87 10-98 



Carbonic anhydride 2-65 2-76 



Phosphoric anhydride trace trace 



Ferric oxide 3-47 3-40 



Ferrous oxide 10-14 10-13 



Titanic oxide trace trace 



Manganous oxide trace trace 



Lime 9-07 9-15 



Magnesia 16-32 16-22 



Potassa : . :'} 0-15 0-10 



Soda '. : .' 0-90 0-93 



99-95 99-89 



Specific gravity 2-88 2-88 



Nickel and chromium were sought for but not found. 



For comparison, I append a series of analyses . of picrite. The 

 first five are given by Tschermak (Sitzungsb: Akad. der Wis- 

 sensch. Wien, vol. liii. pt. 1, p. 260). The next two are ' palseopicrite ' 



* A similar structure, but with the rods more nearly at right angles, is 

 figured as occurring in magma-basalts bv BoricKy, ' Basaltgesteine Bohmens,' 



pi. i. ,;. 



