Reports & Proceedings — Mineralogictil Society. 383 



' A. Brarnmall: "Andalusite (Chiastolite) ; its Genesis, Morphology, 

 and Inclusions." In a survey of thermometamorphic "spotted" 

 rocks evidence based on structural features, optical properties, and 

 microcheruical reactions is adduced to show that certain types of 

 spots convergent towards such minerals as chiastolite, andalusite, 

 cordierite, mica, and chloritoid record arrested development, and 

 that they are probably ontogenetically related. The spot is a 

 complex svstem containing a volatile phase, water, and its develop- 

 ment involves metamorphic diffusion and differentiation controlled 

 by changing conditions of temperature and stress, the tendency being 

 towards the attainment of an equilibrium end-point in a metastable 

 mineral. Thermal and stress conditions adequate to initiate the 

 tendency may be inadequate to sustain it, the time factor also being 

 involved ; development may be arrested and abortive effort recorded 

 as a mineral "spot", the nature of which is determinable, but is 

 often vague or wholly conjectural. The chemical and physical 

 characters of argillaceous sediments are considered with special 

 reference to the genesis of chiastolite. Clays contain a high pro- 

 portion of hydrated silicates of alumina readily soluble and in part 

 probably colloidal. On rise of temperature diffusion effects the 

 segregation of the primary clot ; diffusion inwards of allied molecules 

 and diffusion outwards of alien substances tend to promote homo- 

 geneity and reconstitution within the spot, the peripheral zone being 

 maintained for a time in a relatively high state of hydration. In 

 this connexion the peripheral zone of yellow-brown, non-pleochroic, 

 and isotropic stain is significant ; microchemical tests show that it is 

 due to ferric hydrates, which are known to be liable to spontaneous 

 dehydration, and it is suggested that the ferric hydrate in the 

 peripheral stain acts as a catalyst, assisting dehydration within the 

 spot and transmitting water to the base. For chiastolite (andalusite) 

 a mechanism of formation is suggested to cover the observed facts, to 

 explain the characteristic distribution of its opaque inclusions, and 

 to account for crystals which have the superficial aspect of cruci- 

 form twins. 



R. H. Ha stall : "The Mineral Composition of Oolitic Ironstones." 

 In many oolitic ironstones the ooliths contain more iron or are more 

 highly oxidized than the matrix. Assuming that the iron-content 

 of such rocks is introduced by metasomatism of calcium carbonate, 

 this may be explained in the following way. Many ooliths and 

 organic fragments in limestones consist of aragonite, while the cement 

 is calcite. Aragonite is less stable than calcite and more readily 

 decomposed by iron-bearing solutions, which therefore attack the 

 aragonite first, while the calcite is replaced later. Hence we have 

 the following scheme in successive stages: — 



Ooliths. Aragonite — :> Chalybite — ^- Limonite. 

 Matrix. Calcite Calcite — ?> Chalybite. 



The ooliths are always a stage ahead of the matrix in replacement 

 and oxidation. The origin of the green iron silicate, found in many 

 ironstones, requires further investigation. 



L. J. Spencer : " Eighth List of Mineral Names." 



