W. E. Ford — Optical Study of the Amphiboles. 183 



Figs. 1-4. 



the percentage composition, but, with the data available, it 

 seemed best to assume that this line was straight. It will be 

 noted that the first six analyses form a series, the members of 

 which lie close together. Then there comes quite a break, 

 with analyses 7, 8 and 9 falling again in a group by themselves! 

 There is another break and analysis 10 lies at the end by itself. 

 It is unfortunate that other data were not available by means 

 of which these gaps could have been closed. 



