1 882.] Mineralogy. 927 



breaks it up into sheets, which are re-cemented by prehnite. The 

 prehnite often encloses diabantite, and then varies in color from 

 deep oil-green to jet-black. This black prehnite is sometimes 

 combed out by the slipping rock into long fibres, resembling 

 hornblende or chrysotile. The prehnite also occurs as a finely 

 crystallized double cone or spindle, forming beautiful specimens. 

 This peculiar form is the result of the twinning of three individuals 

 around a common axis, and the resulting optical properties are 

 peculiar. Prehnite also occurs in amydaloidal cavities which are 

 blackened as though held in the flame of a candle. A black sub- 

 stance covers the fibres of prehnite, looking like a net-work of 

 soot-covered cobwebs. This black substance is the result of alter- 

 ation, and is probably chlorophaeite. In other cases the prehnite 

 has changed into a pale-green scaly mass, which appears to be 

 diabantite. 



American Monazites.— In the American Journal of Science for 

 October, Professor E. S. Dana and Mr. S. L. Penfield contribute 

 valuable articles upon American monazites. From a careful 

 measurement of a small monazite crystal from Alexander county, 

 North Carolina, by Professor E S. Dana, the following axial ratio 

 was obtained : 



c (vert) : b : k = 0.95484 : 1.03 163 : t 

 /? = 76 20' 



A table is given containing a list of the more important angles 

 calculated from these data, and agreeing closely with the results 

 of goniometrical measurement. The axial ratio is closely related 

 to that of monazites from other localities. 



Mr. Penfield has analyzed the monazites from Portland, Ct, 

 from Burke county, N. C, and from Amelia county, Va. The 

 latter is the substance originally thought to be an altered micro- 

 lite. Each analysis showed a considerable percentage of thoria, 

 there beino- over fourteen per cent, in the monazite from Amelia 

 county, Va In each case, if the thoria is omitted from the analy- 

 sis, the ratio is obtained of (Ce, La, Di) 2 O s : P 2 O fl - 1 : 1, this 

 being the ratio of a normal phosphate of the cerium metals, 

 R 2 P 2 8 . Moreover, there is just sufficient silica in each analy- 

 sis to make a thorium silicate. Since, therefore, some monazites 

 contain no thoria, and the thoria is here present in varying 

 amount, it is probable that thorium silicate exists as an impurity. 

 That this is indeed the case was proved by examination of a thin 

 section of the mineral under the microscope. _ A dark resinous 

 substance was seen scattered through the ^section. ;md when the 

 latter was moistened with hydrochk 

 posed of gelatinous silica took the 

 the rest of the section being unchai 

 that the thorium silicate is a foreign 

 It probably exists as thorite or orafl 



