G. Palache — Occurrence of Olivine. 495 



The difficulty of securing accurate contact measurements on 

 material of this sort with more or less curved faces is, however, 

 so great that too much weight should not be attached to the 

 discrepancies shown on the olivine side of the table. And it is 

 further to be noted that, of the angles given by Parsons as 

 " measured," only the first between the dominant prism planes 

 could actually have been measured on these crystals since the 

 basal pinacoid is absent, and this first angle agrees equally well 

 with humite and olivine ; the other angles must have been 

 derived from the actual measurements, presumably by halving 

 angles measured over the summit of the crystal between small 

 faces ; such measurements are liable to much greater error than 

 those on larger faces at obtuse angles. On the whole then, the 

 crystallographic evidence alone seems too weak to establish the 

 derivation of the hampshirite pseudomorphs from humite. 



The possibility that the mineral described above as olivine 

 might be humite was carefully considered, especially when the 

 fact was noted that the optical characters of the two minerals 

 are so similar that in granular form they are practically 

 indistinguishable under the microscope. The result of the 

 analysis and the proved absence of fluorine seem to settle this 

 point conclusively. 



Harvard University, June, 1907. 



