54 The American Naturalist. [January, 



zeolitized silicate are accessory constituents. As the amount of the ore 

 increases, that of the other constituents decreases, until in some cases 

 an almost pure magnetite results. By weathering the augite gives rise 

 to mica in abundance. In some cases the pyroxene rock is found asso- 

 ciated with layers in which magnetite and nepheline are the principal 

 components. This Derby 1 believes to be genetically connected with the 

 ore-bearing rocks, which i , and regards as eruptive. 



Much of the ore of the Ipanema district originally occured with acmite 

 and apatite as segregations in an acmite syenite, which is now highly 

 decomposed, so that the segregations are scattered like boulders 

 over the ground. Fouque 2 has redetermined the minerals of the 

 Santorin rocks and has discovered that his former determinations of 

 some of them were erroneous. In the pumice of Acrotri, Isle of Thera, 

 composed of g I material between, are little trans- 



parent crystals whose nature has heretofore been in doubt. A new 

 examination proves them to be alunite. A quantity — separated and anal- 

 yzed—gave: SO s =38#, Al,0,=37.3%; H a O=13.3; alkalies=11.4#. 

 The blocks enclosed in the lava of 1866, formerly supposed to consist of 

 wollastinite, famaU and melanite, are now known to contain in addition 

 to these substances anhydrite. The interiors of the nodules are composed 

 almost exclusively of anhydrite, with a little augite and other con- 

 stituents of the enclosing rock, in which arc wollastnnite and melanite. 

 — Four small boulders of nephrite from British Columbia have been ex- 

 amined by Harrington. 3 Three were found near Lytton on the Fraser 

 River, and the fourth in the upper part of the Lewes River, near the 

 Alaskan boundary line. The composition of each is as follows : 

 Si0 2 A1Q 2 FeO MnO CaO MgO Loss Sp. Gr. 



1. 55.32 2.42 5.35 .52 14.00 20.16 2.16 3.0278 



2. 56.98 .18 4.59 .17 12.99 22.38 2.64 3.003 



3. 56.54 .40 3.61 .16 13.64 22.77 2.92 3.01 



4. 56.96 .51 3.81 .53 13.29 22.41 2.91 3.007 

 No. 3 contains pale hornblende crystals extinguishing at 8°-18°. 



—Derby 4 has discovered that xenotime is an almost universal consti- 

 tuent of muscovite granites, and presumably of other acid potash rock?. 

 Residues obtained by washing the powder of such rocks in a gold 

 washer's pan nearly always yields xenotime and monazite. Eighty-six 

 and two-thirds percentage of the undoubted muscovite granites, from 



1 Amer. Jour. Sci., Apr. 1891, p. 311. 



2 Bull. Soc. Franc, d Min., 1890, XIII, p. 245. 

 3 Trans. Roy. Soc. Can. 1890, p. 61. 



"Amer. Jour. Sci., Apr., 1891, p. 308. 



