Prof. J. W. Judd — On a Common Roch-forming Mineral. 173 



VII. — On the Occurrence, as a Common Eock-forming Mineral, of 

 A Kemarkable Member of the Enstatite Group (Amblystegite, 

 vom Hath). 



By Prof. John W. Judd, F.R.S., Sec. G.S. 



DUEING recent years, much attention has been directed to the 

 importance of the Ehombic Pyroxenes (Enstatites) as rock- 

 forming minerals. In 1879 Prof. Fouque showed that hyjDcrsthene 

 is a common mineral in the Santorin lavas, and in 1883 Mr. Whitman 

 Cross demonstrated that there exists a large and important class of 

 andesites, distinguished by the presence in them of the same mineral 

 hypersthene. Mr. Teall, in the pages of this Magazine, has pointed 

 out that similar lavas exist in this country, which are undoubtedly 

 of pre-Tertiary age; and, now that the attention of geologists has 

 been very generally drawn to the subject, the same rocks have 

 been recognized by many observers in all parts of the globe. 



The class of the Ehombic Pyroxenes, or Enstatites, comprises the 

 bisilicates of magnesia, in which a varying proportion of that 

 base may be replaced by ferrous oxide. This replacement of the 

 one base by the other results in striking modifications in the colour, 

 appearance and optical constants of the mineral, but does not appear 

 to appreciably alter the outward crystalline form. Three chief types 

 of the rhombic pyroxenes have been very generally recognized — the 

 non-ferriferous (enstatite), the slightly ferriferous (bronzite) and the 

 highly ferriferous (hypersthene) ; but much difference of opinion, 

 has existed concerning the definition and limits of these three species. 



Tschermak has proposed to confine the name of enstatite to those 

 rhombic pyroxenes in which the proportion of iron falls below 5 

 per cent. ; to give the name of bronzite to those in which the per- 

 centage of ferrous oxide lies between 5 and 15 ; and to call by the 

 name of hypersthene all those containing more than 15 per cent, of 

 ferrous oxide. 



But there are certain rhombic pyroxenes in which the proportion 

 of ferrous oxide is exceedingly high, ranging from 25 to 35 per cent., 

 so that this base is largely in excess of the magnesia, and in these 

 the physical properties are so entirely different from tliose of the 

 typical unaltered hypersthenes that it seems desirable to give them 

 a distinct name. The most conspicuous character of these ex- 

 cessively ferriferous enstatites, is their very striking pleochroism. 

 The thinnest plates give with the dichroiscope tints varying from 

 a rich red, like that of garnet, to a vivid blue-green. 



Instead of proposing a new name for these very highly ferriferous 

 enstatites, it appears to me to be desirable to revive the old name of 

 " Amblystegite," which was proposed by vom Eath for a mineral of 

 this class in the year 1869. 



I have recently found that this remarkable and very beautiful 

 mineral is by no means uncommon as a rock-constituent. It occurs 

 in rocks of several different classes — quartz-diorites, diorites, dolerites, 

 gabbros, and certain ultra-basic rocks ; and the rocks containing this 

 mineral are certainly very widely distributed, for I have already 



