A point of further interest is that of the age of the rare earth minerals. Except in a few cases where they are obviously of secondary formation, these minerals are among the oldest known to us. They occur usually in igneous rocks, particularly in granites which have been considerably metamorphosed. Where erosion has occurred, they are found in deposits of such a nature as to leave very little doubt that the original rock was of plutonic formation and of very considerable age. Whilst it is true, however, that the rare earth minerals are generally of very great antiquity (none of the primary minerals being of more recent date than the palæozoic age), Eberhard has pointed out that the age and nature of common rocks seem to have absolutely no influence on the traces of scandia and yttria oxides which they contain. The geological evidence shows that the rare earth minerals are on the whole exceedingly stable, and that they have been generally formed during the pegmatitic alteration of granites. As early as the year 1840, Scheerer drew attention to these facts, and to the extreme age of the rare earth minerals; but so far his observation seems to have attracted little attention, and no explanation has been put forward. * * * * * In the following chapters no attempt is made to treat the rare earth minerals fully. An alphabetical list of all the minerals of any importance which contain rare earths, titanium, zirconium or thorium is given, and of these several are selected for fuller treatment. The basis of selection has been somewhat arbitrary. Those species which are of mineralogical importance, as well as those to which any special historical, scientific or commercial interest attaches, have of course been singled out; in addition, the more recently discovered species have occasionally been considered worthy of separate mention.[14] [14] A full list of the minerals containing rare earths known up to 1904, with an account of their properties and very full references, will be found in the work of Dr. J. Schilling, _Das Vorkommen der Seltenen Erden im Mineralreiche_, 1904. It is now being realised that some knowledge of crystallography is essential to the chemist, and for this reason short accounts of the crystallography of the selected types have been given. Apart from this, every effort has been made to render the mineralogy intelligible to the student of chemistry who has devoted no attention previously to this subject, and also to stimulate an interest in the problems of mineral chemistry, unfortunately too often ignored by our present-day teachers. The rare earth minerals afford good examples of some phenomena of great interest to the chemist, as, _e.g._ Isomorphism and Solid Solution, Dimorphism, Isodimorphism, and Molecular Change, and in one or two cases these are treated rather fully.