r'KNTTI FSKOI.A. 



M.-N. Kl. 



which conrniii llu results and (ha.i;iaiiis arrivcfl at by Ford in the most 

 i-cmarl<abk' manner. As they prove with lull evidence, what Frjrd thought 

 probable', that amont^ his f^arnets those niarkecj n:os 8 and 9 show such 

 a great discrepancy with the others that either the analyses or- the refrac- 

 ti\c indices must be ei-i-oneous, 1 omitted them entirely. 



With this improxcd material at hand an attempt was made to con- 

 struct a diagram that could be used IV)r determining garnets by means of 

 tlie réfringence. This was thought [)i-acticable by utilizing all the experience 

 now gathered l)\' me concerning the limits of solid solubility of the various 

 compounds, the conditions of stability and the relations between the che- 

 mical composition of the garnets to the bulk composition and petrologicai 

 chaiMCters of those rocks in which they occur. 



The curve hg. i is based on the following material, all that are not 

 mv analyses (marked with *) having been taken from Ford's table (loc. cit.). 



Here we are concerned only with the garnets of the pyrope-aimandite 

 series. The percentage of MnO, always less than 2, was added to that 

 of FeO, and the sum of (Fe,Mn)0, MgO and CaO was recalculated to 

 100 ^,0. The continuous curve in the diagram expresses the relation be- 

 tween the ratio FeO : (MgO + CaO) and the réfringence. The dotted line 



' Loc. cit. p. 37. 



