May, 1906. Lower California Zoisite — Farrington. 57 



calcium. No further investigation was made of this point, however. 

 Although the formula of zoisite is usually considered to be H s Ca 4 Al 4 

 Si„ 0„, other analysts have obtained percentages which indicate 

 that an additional molecule of water is present. This is true, for 

 instance, of the analyses of zoisite from Fuschthal and Traversella 

 quoted by Dana.* The high temperature required to drive off the water 

 from the Lower California mineral seems to preclude the possibility 

 of its being present as the result of alteration, as might otherwise 

 be assumed. The amount of iron in the zoisite shown by the analysis 

 is high for this mineral and approximates that afforded by epidote. 



The mineral with which the zoisite is associated is, as stated, 

 white and granular. In cavities it exhibits minute imperfect crystals 

 which have a distinct, pearly luster when fractured. The blowpipe 

 and other characters of this mineral indicate it to be prehnite and a com- 

 parison with fragments kindly furnished by Dr. W. T. Schaller leaves 

 little doubt that it is the same mineral analyzed f by him and found 

 to be prehnite. Its association with zoisite is of interest owing to the 

 similarity in composition of the two minerals. The prehnite seems 

 generally to furnish a matrix which the zoisite penetrates, but occa- 

 sionally it coats the zoisite groups in such a way as to suggest that it 

 is an alteration product of the latter. The unusual features of the 

 zoisite seem to be therefore, its radiating habit, its high content of 

 water and iron and its association with prehnite. 



Through the kindness of Prof. L. P. Gratacap of the American 

 Museum of Natural History, ths writer was permitted to study two 

 specimens of zoisite in the collection of that institution which were 

 undoubtedly from the same locality as the above. They have the 

 more usual ash-gray color of zoisite and the grouping of the crystals into 

 cones is only partial. For the most part the crystals occur in hemi- 

 spherical cavities which were, in the specimens studied, about three 

 inches in diameter. The crystals interlace these cavities with great 

 variations of size and direction. Many of the crystals are quite 

 minute. All are from acicular to bladed in habit. Although some 

 crystals have free terminations, no end faces could be discerned. 

 These specimens show that grouping into cones is not constant for the 

 zoisite from this locality but its occurrence at all is noteworthy . 



* System of Mineralogy, 6th ed., p. 514. 

 tBull. U.S.Geol. Survey No. 262, p. 128 



