146 AXXALS XEW YORK ACADEMY OF HCIEyCES 



In other parts of the slide, the amphiboles are a pale green to deep green. 

 In several places in 94, groups of very pale green crystals are seen. In 

 120, 100 and 51a, similar, almost colorless crystals are observable. 



Among the several varieties of amphibole present in these rocks, the 

 arfvedsonite is believed to have been first in order of formation, the 

 columnar prisms with large extinction angle second, the fibrous form 

 resembling actinolite third and the colorless variety fourth. 



The changes noted in the amphiboles conform to those which a complex 

 solid solution might be expected to undergo when the external conditions 

 are continually changing. The relations of amphiboles among themselves 

 and to pyroxenes are undoubtedly very complex and are not well under- 

 stood, but the researches which have recently been carried forward in the 

 Geophysical Laboratory at ^\'ashington have cleared up a number of 

 hitherto doubtful matters. 



In a study of the nature of the MgSiOg series of minerals, E. T. Allen, 

 F. E. Wright and J. K. Clement*^ found that MgSiOg exists in four 

 modifications or phases : a monoclinic and an orthorhombic pyroxene and 

 a monoclinic and an orthorhombic amphibole, and that the last three pass 

 into monoclinic pyroxene at high temperatures (1150° and upward) with 

 a slight evolution of heat. They draw the conclusion that all are mono- 

 tropic toward the monoclinic pyroxene, which is the only stable phase at 

 all temperatures. Somewhat opposed to this inference, however, is the 

 fact, which they and other investigators record, that monoclinic amphi- 

 bole is produced experimentally by heating the necessary ingredients at 

 moderate temperatures (375-475°) for 3 to 6 days in aqueous solutions; 

 also the numerous examples in which the transformation of pyroxene into 

 amphibole has been observed in nature (as in uralitization). They re- 

 gard natural occurrences of amphibole as examples of the persistence of 

 metastable phases, but this explanation seems hardly adequate to account 

 for all the phenomena which petrographers have observed. A factor 

 which is undoubtedly of great importance in the matter is that no member 

 of the series ordinarily occurs pure in nature, but contains other members 

 associated with it either in chemical combination or in solid solution. 

 This may exert a powerful effect upon the direction of transformation 

 under given conditions. 



Further study in the same laboratory on "Diopside and its Relations to 

 Calcium and Magnesium j\Ietasilicates" *'' has brought out very interest- 

 ing relations. The investigation was devoted to pyroxenes and did not 



" Am. Jour. Scl., 4th ser., vol. 22, p. 385, 1906. 



*»Allex, White, Wright and Lausex : Am. Jour. Sci., 4th sen, vol. 27 pp 1-47 

 1909. 



