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University of California. 



[Vol. i. 



characteristic spindle-shaped crystals of fluosilicate of calcium. 

 As certain varieties of olivine contain lime, it was necessary to 

 determine whether alumina was present or not, prehnite being a 

 lime alumina silicate, while olivine contains no alumina. For this 

 purpose Behren's method was used. A few very minute grains 

 were obtained, and, after being reduced to a powder, were treated 

 with hydrofluoric acid. This decomposed the mineral, and upon 

 evaporation a few drops of pure sulphuric acid were added and the 

 mass reduced to a sulphate. After evaporation the residue was 

 diluted slightly with water and the solution was ready for examina- 

 tion. A microscopic examination showed it to be filled with minute 

 rod-like crystals of gypsum. A drop of caesium sulphate was added 

 to part of the solution on a glass slide. At first no reaction took 

 place, but upon standing a day numerous small crystals of caesium 

 alum separated out, having the form of very perfect octahedra. 

 This reaction showed that the mineral was not olivine, and inas- 

 much as lime was abundant and magnesia not definitely recognized, 

 the determination of the mineral as prehnite was considered as cor- 

 rect. As a check upon the above, the specific gravity of several 

 grains was determined by Klein's solution and found to be 2.894, 

 the specific gravity of prehnite ranging from 2.80 to 3. 



Many of the analcite areas contain large numbers of the preh- 

 nite individuals, only a part of which are idiomorphic, and in rare 

 instances a single crystal was observed nearly filling a wedge-shaped 

 area (Plate 15, Fig. 6), being partially allotriomorphic. The perfect 

 idiomorphic boundaries of those individuals lying farther within the 

 analcite, and the more irregular granular form of the smaller ones 

 clustered near the outer edge, and their lack of attachment to the 

 feldspars bounding the analcite, suggest very strongly their forma- 

 tion within the analcite and not in a cavity. Many instances appear 

 of prehnite crystals wholly separated from the feldspar by the anal- 

 cite, and in some cases by secondary feldspar. The green fibrous 

 material eating into the analcite has slightly affected a portion of 

 the prehnite crystals. 



The question may arise as to whether the idiomorphic outline 

 of many of the prehnite individuals does not make their formation 

 from the analcite impossible. It might seem more likely that they 



