H. S. Washington — Rhyolites of Lipari. 451 



LlTHOIDAL RHYOLITE. AeOLAL LlP AROSE ■ (1.4.1.3) . 



At the southern end of the island of Lipari the massif 

 of Monte Guardia (including Monte Giardina), is com- 

 posed very largely of rhyolites of a type that is very 

 different from the later obsidians and pumices, though 

 obsidians are also met with here. These are what have 

 been called lithoidal rhyolites, the liparite of von Richt- 

 hofen. They have a dull luster in the hand specimen, 

 that is, are megascopically felsitic, though they may be 

 actually highly vitreous as seen in thin section. For the 

 present it will suffice to describe both the hyaline and the 

 noncrystalline varieties together, since they both show 

 the same, or almost the same, megascopic features. The 

 description is based on two specimens of this type which 

 were analysed, and which appear to be characteristic of 

 these rocks on Lipari. The term aeolal is derived from 

 the common name of the island group, the Aeolian 

 Islands, and is applicable especially to the noncrystalline 

 type. 



Megascopic characters. — One specimen, from near the 

 southern end of Monte Guardia, is of a pure light ash- 

 gray color, dull luster, and very fine-grained — a typical 

 felsite. There are some small phenocrysts of a greenish 

 black pyroxene, some of them surrounded with very thin 

 zones of white feldspathic material, but there are prac- 

 tically no feldspar phenocrysts. A decided flow structure 

 is shown by very narrow, whitish streaks. The other 

 specimen, from a small quarry on the northwest slope of 

 Monte Giardina (north of Monte Guardia), is also very 

 fine-grained and of a dull luster. Its color is a very light, 

 slightly brownish gray. There are no phenocrysts, but 

 a banded structure similar to that of the Monte Guardia 

 specimen is evident. 



Microscopic characters. — In thin section the Monte 

 Guardia specimen shows a quite typically trachytic 

 texture, and appears to be noncrystalline. There are 

 rather numerous small prismoids of almost colorless 

 augite, with very few small phenocrysts of orthoclase, 

 in a finely "felted" groundmass, made up of very small 

 laths of orthoclase, few magnetite grains, and with what 

 appears to be some interstitial quartz. If glass is 

 present, it is in very small amount and is not easy to 

 detect. Flow structure is marked. 



