Cerite contains from 59·4 to 71·8 per cent. of rare earths (oxides), the amount and nature of which vary with the precise locality. The oxides consist chiefly of ceria, lanthana, and didymia (praseodymia and neodymia), the complexity of the so-called ceria having been shown by Mosander in the case of ceria separated from gadolinite as well as from cerite; but yttria earths are also found to a small extent in the mineral. It is remarkable that neither thorium nor uranium has been found in cerite, which is thus practically unique among the rare earth minerals. This anomaly becomes even more marked in view of the very high percentage of inert gases found by Tschernik[23] in a related mineral from Batoum. This is a very complex mineral in which the basic part is represented by rare earths, chiefly ceria earths (50·8 per cent.) with water (3·4 per cent.), and oxides of iron, calcium and copper (6·8 per cent.); the acidic oxides being silica (6·6 per cent.), zirconia (11·6 per cent.), and titanium dioxide (14·7 per cent.), with phosphorus pentoxide (3·2 per cent.), and sulphuric anhydride (1·7 per cent.). Traces of thoria are present, but no uranium; very considerable quantities (up to 1 per cent.?) of helium were found. [23] G. Tschernik, _J. Russ. Phys. Chem. Soc._ 1896, ~28~, 345; 1897, ~29~, 291. Abstracts in _Zeitsch. Kryst. Min._ 1899, ~31~, 513 and 514. It is somewhat heavier than cerite (sp. gr. 5·08), but otherwise resembles it closely. ~Gadolinite~ (Ytterbite).--Gadolinite is a silicate of iron, beryllium, and the yttria earths, of the formula 2BeO,FeO,Y₂O₃,2SiO₂, which may be written FeBe₂Y₂Si₂O₁₀. According to Groth, it is a basic orthosilicate, Be₂Fe(YO)₂(SiO₄)₂, derived from the acid H₈Si₂O₈. The beryllium content varies considerably, and some authors recognise two varieties of the mineral, one rich, and one poor in beryllium; but Scheerer pointed out in 1840 that iron and beryllium are probably vicarious constituents. Analysis gives silica 21·8 to 25·3 per cent.; yttria earths 22 to 47 per cent.; ceria earths 5 to 31 per cent. In a variety from Ytterby, the rare earth Scandia was first found, forming up to 0·02 per cent. of the mineral. Small quantities of thoria, ThO₂ may be present, and traces of helium were found by Ramsay, Collie, and Travers. According to Strutt it contains also uranium and radium. Like cerite, it does not often occur crystalline, being usually found in amorphous masses. The crystals are monoclinic; _a_ : _b_ : _c_ = 0·6273 : 1 : 1·3215; β = 89° 26¹⁄₂´. Common forms are--Ortho-, clino-, and basal pinakoids, _a_ {100}, _b_ {010}, and _c_ {001}, hemi-prisms _m_ {110}, _v_ {120}, clino-prisms _w_ {012}, _q_ {011}, and many others; and various hemi-pyramids {hkl} and {h̅kl}. Angles _a_ ∧ _m_ = 32° 6´, _c_ ∧ _q_ = 52° 53´, _c_ ∧ (101) = 64° 9´.