76 Dr. H. J. Johnston-Lavis—Trachyte at Naples. 
have much the form of a pyroxene with the faces 110, 100, 112 and 
111 well developed. A few were sliced, and show under the 
microscope a yellowish-green mineral, changed on the surface and 
along the cracks to a dark almost opaque brown mass. Unfortunately 
no very characteristic cleavage or extinction angles could be obtained. 
A grain reduced to powder and fluxed with carbonate of soda gave 
a strong manganese reaction. These facts would seem to indicate 
that we have pyroxene crystals which are undergoing decomposition, 
either from the separation of an oxide or hydrated oxide of 
manganese, or the introduction of this from without. The former I 
take to be the correct explanation, since the brown stains only occur 
on these crystals and in their immediate neighbourhood, and also the 
amphibole rods are unaltered except where in contact with these 
pyroxene crystals. | 
In some of the vesicles are bunches of hair-like crystals of 
chocolate colour, which at first sight look like breislakite. Hxamined 
under the microscope they are seen as absolutely opaque rods, even 
to the smallest dimensions. Although easily visible as large sprays, 
the actual amount of material is very small, so that I was unable 
to obtain more than two or three milligrammes. Part of this was 
treated by the four acids, but proved very slightly soluble in 
HCl and HF. 
The HCl solution gave a blue precipitate with K,CyFe,. 
Another portion was fluxed with alkaline carbonates and proved 
very refractory, lixiviated with subsequent addition of HCl 
evaporated to dryness, then, dilute HCl added and filtered. The 
filtrate gave strong iron reactions, whilst the filter was burnt, and 
the ashes treated with HF and H,Go, evaporated and fluxed with 
PotBisulphate and a HCl solution made which gave faintly the 
reaction for titanium. With salt of phosphorus in reducing flame 
it gave a blood-red bead. From these reactions we may conclude 
that we have to deal with a compound containing principally 
titanium and iron, and the question remains open whether we have 
to deal with a fibrous hematite containing titanium (Scacchi has 
proved the fibrous mineral in the piperno to be an iron-oxide), or 
a fibrous rutile very rich in iron, or lastly perhaps a breislakite 
containing titanium. 
Most of the cavities are completely lined by a carpet of minute 
sanidine crystals; but superposed on these, in a few, are small 
elongated hexagonal-looking prisms which rarely attain to more 
than half a millimetre in length and about a sixth in breadth.’ 
Under the microscope they are larger in the centre, so as to appear 
fusiform, whilst the surfaces of the prism are striated longitudinally, 
so that in only a few cases can they be made out to be hexagonal. 
The prism is terminated sometimes by basal planes, but in others by 
a low pyramid. 
Various attempts were made to measure the angle between the 
pyramid and prism, and most often with a resulting angle of 116°, 
' Often poised delicately on the amphibole rods, or the fibres of the breislakite- 
like mineral. 
