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University of California. 



[Vol. 2. 



paratively large masses in the crystalline schist. Though entirely 

 amorphous, some fragments show suggestions of a scaly structure, 

 and in such cases have a mother-of-pearl luster. Material from a 

 boulder obtained from locality A, which was selected as carefully 

 as possible but was probably not absolutely free from very fine 

 actinolite crystals, gave the analysis recorded in the fourth column 

 of Table V. The analysis is remarkable in the comparatively low 

 magnesia and the very high iron and alumina; in fact, in both of 

 the latter figures it exceeds any of the analyses of this mineral of 

 which I have record. The analysis, together with the scaly 

 structure previously alluded to, suggests that the mineral has 

 been derived from chlorite. 



DIALLAGE. 



This forms a light-colored friable rock which is of rather fre- 

 quent occurrence in both of the localities here considered, being 

 found both as detached fragments and as groups of massive boulders, 

 the latter probably in situ. The specimen examined was obtained 

 from locality B. Structurally the rock is composed entirely of 

 flakes or lamellae, which often present several square centimeters of 

 surface and are distinguished by their greenish-yellow color and 

 dull, non-lustrous character. True crystallographic planes were 

 never observed, but the lamellae were often divided by a series of 

 more or less pronounced cleavages into prismatic fragments, some- 

 times suggesting a fibrous structure. The lamellae show parallel 

 extinction and with convergent light the eccentric emergence of an 

 optic axis, but a section cut approximately parallel to the prismatic 

 cleavage gave an extinction angle of 7 . It is clear then that the 

 lamellae represent orthopinacoidal cleavage flakes of a monoclinic 

 pyroxene. A series of transverse partings traverse many of the 

 lamellae, but there is no visible evidence of alteration, nor were 

 inclusions of other minerals observed. The interference colors are 

 of a high order. 



The chemical composition of this substance, as shown by 

 analysis in Table VI, is clearly that of diopside, while its structural 

 features class it as diallage. It shows a remarkably close agreement 

 with a sample of diallage from Scotland, analyzed by Heddle, the 



