256 JV. L. Bowen — The Ternary System : 



ties with the artificial pyroxenes. The analogy extends to 

 some of their other relations. For example, Wahl has shown 

 that, in a great variety of rocks bearing these enstatite-augites, 

 the earlier pyroxene crystals are more magnesian, the later 

 more calcic. This is sometimes true of the earlier and later 

 parts (zones) of the same crystal, a fact which has been pointed 

 out above for the artificial crystals. 



It would appear, then, that pyroxenes more or less similar to 

 the artificial lime-magnesia pyroxenes (differing from them 

 principally in their iron content) have considerable importance 

 in igneous rocks. 



It has been shown in the optical part of this paper that, for 

 the artificial lime-magnesia pyroxenes, the composition can be 

 determined fairly closely from the optical properties. The 

 ability to do so for the natural pyroxenes of rocks is very 

 desirable and the present results may be considered a step in 

 that direction. The lime-soda feldspars are at present the only 

 isomorphous series of rock-forming minerals in which chemical 

 composition and optical properties can be correlated with fair 

 accuracy. Among recent studies in this direction that of Ford 

 on the amphiboles may be noted. Until a great deal of such 

 work has been accomplished, the determination of the propor- 

 tions of minerals in a rock, as by the Rosiwal method, can, in 

 the usual case, have little value. 



Resorbed Olivines in Natural Rocks. 



It has been shown that in the artificial melts discussed, from 

 which olivine is the first mineral to crystallize, the olivine is at 

 a later stage either partly or completely redissolved, giving 

 pyroxene by reaction with the liquid. This resorption takes 

 place in the normal course of crystallization on cooling and is 

 the necessary result of equilibrium between the phases. In 

 natural rocks the resorption of olivine often takes place, an 

 example occurring in the case of the Palisade diabase of New 

 Jersey, where, in the contact facies, olivine with reaction rims 

 of enstatite is observed.* Such resorption has sometimes been 

 assumed to imply some such drastic change as the sudden 

 relief of the pressure to which the magma was subjected and 

 the consequent instability of minerals formed under the higher 

 pressure. Minerals exhibiting resorption are, moreover, some- 

 times assumed to have formed from a magma having a com- 

 position different from that of the rock in which we find them 

 and, perhaps, to have sunk into a magma which exerted a 

 solvent action upon them. In the case of olivine such assump- 



* Lewis, J. V., Ann. Rep't, State Geologist, New Jersey, 1907, 133. 



