34 



University of California. 



[Vol. i. 



and parallel to the c axis, and that the acute bisectrix is perpendic- 

 ular to the cleavage, being coincident with the a axis. The cleav- 

 age plates exhibit a pronounced fibrous structure parallel to ]>. In 

 these plates and in all sections transverse to the cleavage in the 

 slides, the extinction is strictly parallel to the cleavage, to the trans- 

 verse fibrous structure, and to the traces of the pinacoids. This 

 shows that the three axes of elasticity are parallel to the three crys- 

 tallographic axes respectively, and that the mineral is therefore 

 orthorhombic. 



Since the plane of the optic axes lies in the brachypinacoid 6 = 2. 

 By the aid of the quarter-undulation mica plate, in sections parallel 

 to the cleavage, and the quartz wedge in sections parallel to the optic 

 axial plane, it was ascertained that the crystals were optically neg- 

 ative. Therefore the axis of maximum elasticity is the acute bisec- 

 trix, so that Ct = ct, and t—'c. 



In thin section iddingsite becomes transparent in colors which 

 range from a deep chestnut brown to citron yellow, or occasionally 

 a clear yellowish green. The pleochroism is strongly marked in 

 sections transverse to the cleavage, particularly so in those parallel to 

 the optic axial plane, but is usually very feeble in sections parallel 

 to the cleavage. The absorption formula is t > 6 > a. 



The interference colors, between crossed nicols.for sections par- 

 allel to the cleavage (macropinacoid) do not differ perceptibly from 

 those observed with the analyzer removed, except that a slight 

 decomposition of the light due to the fibrous structure becomes 

 apparent. In the most favorable sections parallel to the optic axial 

 plane, however, interference colors are bright green and carmine 

 tints, almost like those of muscovite, to which resemblance the 

 eminent cleavage adds greatly. The double refraction is thus 

 strong. The mean index of refraction is low. 



No mineral answering; to the combination of characters above 

 detailed can be found described in the books. The external form of 

 the crystals at once suggests that of olivine. But the chemical, 

 optical, and other physical characters separate it absolutely from 

 olivine. And the only question in this connection which can arise 

 is, whether or not iddingsite is a pseudomorph or alteration prod- 

 uct of olivine. It seems to us that the evidence is far from con- 



