\ 
454 Hague and Iddings—Rocks of the Great Basin. 
_most important advance made lies in the direction of more 
accurate methods for determining the species of feldspars. 
Tnvestigations in recent years have shown that many feld- 
be 
more species of triclinic forms. This is noticeably the case 
with the rocks of Hungary and the volcanic islands of the 
Mediterranean, like Santorin. In consequence many - lavas 
formerly determined as trachytes are now more properly refer- 
red to andesites. Again, since Zirkel’s work a large number 
of new thin sections have been prepared from the rocks not 
heretofore examined in the collection of the Fortieth Parallel 
