E. S. Dana — C/iondroilitefrom the TiUy-Foster Iron Mine. 71 



Tlie smallest crystals proved to be free from this cause of irregu- 

 larity, and one of tliem, on which the faces were exceptionably bril- 

 liant, was chosen for careful measurement. It may be added that all 

 the nieasui-cnients were made Avith an Oertling goniometer, pi-ovidod 

 with two telescopes. 



The mean of 30 measurements of A {0=.00\) on r' (—-2^247) 



gave: 135° 18' 50". The maxinumi variation from the mean given 



was ±45". The mean of 30 measurements of ^ on (i"|— -^=z205 J 



gave: 140° 55' 48". Maximum variation -4-45". 



These were accepted as the fundamental angles, and as the agree- 

 ment l)etween the other anoles measured and the calculations made 



Table I. 

 Ghondrodite. 



Hwmite. 



upon the above basis proved to be as close as could be desired, hav- 

 ing regard to the nature of the plane in each individual case, no 



