40 CALIFORNIA ACADEMY OF SCIENCES. [PROC. 30 SER. 



the frequent cracks, and appear to be due to limonite formed 

 by oxidation of the contained iron. In all the attempts made, 

 however, this color could not be leached out by acids. The 

 deep color of the sections and strong absorption of light 

 prevented an entirely satisfactory determination of some of 

 the optical properties. 



The pleochroism is marked in sections transverse to the 

 cleavage, but is not so strong in sections showing no cleav- 

 age. The absorption formula is c > 6 > a. The mineral 

 possesses a rather low mean index of refraction. In the 

 thinner sections the iddingsite may be seen to possess a 

 strong double refraction, though the polarization colors are 

 usually masked by the deep color of the mineral. In sec- 

 tions parallel to the plane of cleavage, though the transverse 

 fibration parallel to b is distinct, the color of the mineral 

 conceals the fibration at right angles to this. 1 



No satisfactory material could be obtained for investi- 

 gating the mineral chemically, nor was any attempt made 

 to analyze the rock as a whole, on account of the secondary 

 silica contained in it. One of the slides was uncovered and 

 an attempt was made to stain the mineral. This was suc- 

 cessful only after several trials had been made, both with 

 concentrated and dilute acids. Dilute boiling sulphuric 

 acid finally caused the mineral to take the stain, the results 

 thus agreeing with those obtained by Dr. Ransome. 2 



No definite information was obtained from the Santa 

 Catalina specimens as to the origin of this mineral. Its oc- 

 currence in a rock of this type, and possessing the form 

 characteristic of olivine, would certainly, in the absence of 

 any evidence to the contrary, point to the strong probability 

 of its being a pseudomorph after that mineral. If the min- 

 eral described by Iddings 3 is the same as that under discus- 

 sion as it appears to be it leaves little doubt on the ques- 

 tion. 



iSee "The Eruptive Rocks of Point Bonita," by F. Leslie Ransome. Bull. Dept. 

 Geol., Univ. Cal., Vol. i. No. 3, p. 91. 



*Loc. cit., p. 92. 



8 "Geology of the Eureka District, Nevada." Monongraph XX, U. S. G. S., Appen- 

 dix B, pp. 388-390. 



