The structures of the silicates which occur in nature present a highly interesting series of problems for X-ray analysis. The number of crystalline forms is very large, and they have been investigated very thoroughly as regards their crystallographical and optical properties, and the way in which isomorphous replacement of one constituent by another takes place. It is generally possible to obtain well-developed crystals which are suitable for purposes of X-ray investigation. The difficulty of analysis is due to the complexity of the molecules which enter into their composition, and the low symmetry of most of the crystals; a direct attack on the problem of their structure by the usual type of X-ray examination is not easy. There are, however, certain features of these compounds which reduce the difficulties of analysis, and enable an insight into the structure to be gained even when its complete determination is not possible. The especial feature which is discussed in the present paper is the fundamental part played by the oxygen atoms in the crystalline arrangement. The structure may be regarded as an assemblage of oxygen atoms, cemented together by silicon and by metallic atoms. In the case of most metals which are of common occurrence in the silicates, their insertion between the oxygen atoms causes only a slight distortion of the oxygen arrangement, and the relative proportion of these metals and of silicon can be varied within wide limits provided that their positive valencies neutralise the negative valencies of the oxygen atoms.