152 H. 'S. Washington — Khyolites of Lipari. 



The specimen from Monte Giardina is quite different 

 in thin section. It is very largely composed of a color- 

 less glass, thickly crowded, however, with very small 

 orthoclase prisms, angite prismoids, and fewer minute 

 magnetite grains. There are also present, in consider- 

 able quantity, small, yellowish spherulites, to which the 

 slightly brownish color of the hand specimen is doubtless 

 due. Flow structure is also well developed. 



Chemical characters. — My analyses of these two speci- 

 mens, with one by Abich of a Monte Guardia lithoidal 

 rhyolite, are given in Table II. 



TABLE II. 



12 3 



SiO, 74.70 69.64 68.35 



ALO, 12.71 13.48 13.92 



FeXK 1.15 1.93 2.28 



Feb 0.28 1.37 n.d. 



MgO 0.07 1.68 2.20 



CaO 0.54 2.80 0.84 



Na.O 3.89 4.05 4.29 



K.0 4.91 4.87 3.24 



HX>+ • 1.01 0.17 ) 



[ 4.64 



H,0— 0.10 0.12 ) 



TiO, 0.10 0.13 n.d. 



P,0- n.d. 0.09 n.d. 



MnO 0.04 trace n.d. 



99.50 100.33 99.76 



1. — Lithoidal rhyolite (dohyaline), Monte Giardina, Lipari, Washington 

 analyst. 



2. — Lithoidal rhyolite (holocrystalline) , Monte Guardia, Lipari, Washing- 

 ton, analyst. 



3. — Lithoidal rhyolite, Monte Guardia, Lipari, Abich analyst. Abich, Vul- 

 kan. Ersch., 1841, p. 25. 



The dohyaline Giardina rhyolite resembles very closely 

 the obsidians described above, the only marked point of 

 difference being in the relative amounts of the iron 

 oxides. Here ferric oxide is present in much greater 

 amount than ferrous. The Guardia rhyolite is decidedly 

 lower in silica, but higher in iron oxides, magnesia, and 

 lime than the obsidians, though the alkalies are about 

 the same. In this holocrystalline rock ferric oxide is 

 present in considerably greater amount than ferrous. 



Classification. — The norms of the rhyolites from Giar- 

 dina and from Guardia, as analysed by me, are calculated 

 to be as follows : 



