280 A TREATISE ON METAMORPHISM. 



But, even if this be true, it is freely admitted that the case is not fully 

 covered, for it is' very uncommon indeed for the chief resultant mineral of 

 an alteration in the zone of anamorphism to have a lower specific gravity 

 than the minerals from which it is derived with comparatively small chem- 

 ical change. Apparently, for some reason the amphiboles are more stable 

 under conditions of moderately deep-seated metamorphism than the pyrox- 

 enes. This view is confirmed by the fact that, while the majority of the 

 schists and gneisses are amphibolitic rather than pyroxenitic, in some of the 

 gneisses and schists which have been altered under very deep-seated condi- 

 tions the pyroxenes are present instead of the amphiboles. The significance 

 of this fact is probably that an unusually high pressure is required in order 

 to produce the mineral of the highest specific gravity in the case of the 

 pyroxene-amphibole group. 



The change from augite tobiotite, equations (10) and (11), is one which 

 takes place in the zone of anamorphism especially under conditions of 

 mashing. In this change the volume of the biotite produced is greater 

 than that of the pyroxene; in the case of the equation (11) 17.26 per cent. 

 However, this case is similar to that of hornblende. Potassium salt must 

 be added from some other mineral and a calcium salt is produced. In 

 order to get the real volume relation of the reaction it would be necessary 

 to know the source of the potassium and the place to which the calcium 

 goes; and as present information does not enable us to determine this, no 

 definite statement can be made as to the total effect of all the changes 

 involved in the alteration of augite to biotite. 



Alterations of pyroxenes other than the diopside-augite series. No equations are Written 



for the alterations of wollastonite, hedenbergite, acmite, and pectolite, 

 because the character of the alterations of these compounds has not been 

 described in the standard authorities, although there is no doubt that these 

 minerals, like all others, do imdergo various alterations. All these minerals 

 form under deep-seated conditions; and it is to be expected that under the 

 conditions of the zone of katamorphism, especially in the belt of weathering, 

 they would be decomposed; but, if so, the minerals into which they change 

 are unknown. 



Alterations of spodumene are recorded. According to Dana, the first 

 stage in the alteration of spodumene is to beta-spodumene (crystallization 

 not determined; sp. gr. 2.644-2.649), in which one-half of the lithium is 



