133 ANNALS NEW YORK ACADEMY OF SCIENCES 



which the combinations created by consolidation did not present the condition 

 of most stable equilibrium ; especially does this apply to the basic plagioclases, 

 whose silicates tended toward a breaking-up into soda-aluminum silicate (al- 

 bite) on one hand, into lime-aluminum silicate (zoisite, epidote) on the other. 



In this citation, Becke brings out clearly the conception that the sta- 

 bility of a mineral compound depends upon the conditions surrounding 

 it, and that a compound which is stable under the conditions of its forma- 

 tion may become unstable under later conditions, and, if opportunity is 

 afforded, will recrystallize in stable form. The writer wishes to empha- 

 size this point on account of the manner in which a mineral is often 

 spoken of as being very stable or very unstable. Unless the surrounding 

 conditions are known, these terms have little meaning, for a reaction 

 which proceeds in a certain direction under given conditions will (gener- 

 ally) proceed in the opposite direction under a reversal of conditions. 

 This is very clearly expressed by C. R. Van Hise, who, in his work on 

 "Metamorphism,"*^ refers repeatedly to the manner in which reactions 

 occurring in the zone of katamorphism are reversed in the zone of ana- 

 morphism, but many writers appear to ignore the principle. Through- 

 out the present paper, one of the chief objects which has been kept in 

 view is to show how quickly minerals respond to an alteration of external 

 conditions, and express this facility of response in recrystallization, if 

 opportunity is given. 



Hintze*^ gives numerous additional illustrations of the formation of 

 secondary albite. In the vein-granite of the Eiesengebirge, it is found in 

 druses in the form of clustered rosettes or cockscomb crystal-aggregates. 

 At Striegau, it covers walls of vugs and at times rests upon potash feld- 

 spars ; in the Harz, it occurs in beautiful crystals in gabbro ; near Mar- 

 burg, Hesse, it forms a later deposit in crevices in diabase. 



These various illustrations of the occurrence of secondary albite are 

 cited to show its relations to the feldspars of the original rocks and are 

 believed to confirm the view held by the writer that the various molecules 

 which enter into feldspars are chemically independent and that the anor- 

 thite molecule (in some cases also the orthoclase molecule) may enter 

 new combinations without affecting in any manner the stability of the 

 albite molecule. 



Quartz 



Unlike albite, whose crystals are generally minute, quartz is often 

 prominent in hand-specimens. The crystals frequently attain a thickness 



" Monograph 47, U. S. Geol. Surv., pp. 170, 172, 176, and especiaUy 364-368. 

 *» Op. cit., pp. 1433-1454. 



