THE ANALCITE BASALTS OF SARDINIA 753 



The presence of considerable biotite in the rocks of Binzale 

 Prunu and Ghizo is so striking that they may be considered to form 

 a special type of analcite basalt, which may be called ghizite, if a 

 special name be deemed advisable. 



The occurrence of analcite in these rocks, as well as in those of 

 Colorado, Ireland, and elsewhere, which, without chemical analysis, 

 can only with great difhculty be distinguished from leucite, so that 

 the rocks are on superficial examination considered to be leucite- 

 bearing, indicates the necessity of chemical analysis, and the advis- 

 ability of a revision of some of the occurrences of so-called leucitic 

 rocks. This is especially true of certain regions, such as Bohemia, 

 Kula,^ and Trebizond, where basalts containing a mineral supposed 

 to be leucite occur, but where the rocks and the regional magmas 

 are eminently sodic rather than potassic. As a general rule, as is 

 well known, analcite is apt to occur in the rock base as the last, or 

 one of the last, products of crystallization, but its undoubted occur- 

 rence in the instance described in this paper and those occurrences 

 cited in the table of analyses show that its presence in leucite-hke 

 phenocrysts may be much more general than has been hitherto 

 supposed, and may clear up some of the obscurities surrounding 

 the occurrence of leucite.^ 



' It now seems highly probable, in view of the present study, that the supposed 

 leucites of the "leucite" kulaites of Kula are in reality apalcites. Though the potash 

 is higher than in the rocks cited above, yet they are dominantly sodic, and the sup- 

 posed leucites rarely show optical anomalies. 



' Cf. H. S. Washington, Jour. GeoL, XV (1907), 277. 



