THE ANALCITE BASALTS OF SARDINIA 745 



the groundmass circular areas which are clear and colorless. The 

 diameter of these varies somewhat, in those of Scano being from 

 I to 2 mm., while in that from Binzale Prunu it is never over o . i mm. 

 These are almost exactly the same as the small leucites of the leucitic 

 lavas of the Italian Peninsula. The boundaries are more or less 

 well defined, often by a circular line of small grains. They contain 

 the ordinary inclusions common to leucite, of augite and magnetite 

 microlites, which form either a central core or a circular wreath. 



Fig. 2. — Analcite basalt, Monte Columbargiu 



They are never radially arranged. All these leucite-like crystals 

 are absolutely isotropic, even when the sensitive tint is used, and 

 no optical anomalies or twinning phenomena are seen between 

 crossed nicols. The observations of Dr. Wright showed that these 

 crystals have a refractive index of i . 502, which is lower than that 

 of leucite and above that of analcite. 



The microscopic groundmass is composed of abundant, very 

 small prisms of augite and fewer anhedral grains of olivine and 

 magnetite, scattered without any definite arrangement in an 



