Notices respecting New Books. 515 



normal crystalline system ; he refers also to the concretions in the 

 Magnesian Limestone of Durham. 



The vitrification and devitrification of rock {e.g. obsidian and 

 tachylite) belong to metatropy. Sulphur and phosphorus, each 

 being allotropic, are taken in illustration, as also arsenious oxide, 

 metaphosphoric acid, silica, borax, and calcium fluoride. The 

 author states that with regard to devitrification the following in- 

 ductions seem to be warranted: — 1. The vitreous state of a body 

 represents a more primitive and simple molecular structure, with a 

 corresponding low degree of stability. 2. There is a latent heat of 

 vitrification, the loss of this being accompanied by the building up 

 of more highly complex and more stable molecules, with a tendency 

 to assume a crystalline form. 3. In some cases hydration or de- 

 hydration, as the case may be, appears to be a factor in the process 

 of devitrification. 



The behaviour of artificial glasses under various conditions is 

 next considered with respect to devitrification, also the behaviour 

 of flint under certain conditions. 



Viewing the relative densities of the different allotropic forms 

 of the same mineral, the author states that, as the maximum 

 density and stability of molecular structure is identified with the 

 crystalline form, pressure, by way of compression, is favourable to 

 crystallization. Pressure crushing a rock may allow freer access 

 and circulation of water with minerals in solution, and so prepare 

 the way for paramorphic changes, but cannot induce direct meta- 

 tropic change — therefore, continues the author, mere deformation 

 of rocks by pressure may have had too much attributed to it as a 

 factor in metamorphism. 



The passage of minerals through this solid-liquid " critical state " 

 (i.e. the state in which they were neither solid nor liquid), as 

 exemplified by certain flattened masses of quartz, is illustrated 

 with facts observed by the author and others. The access of water 

 to rocks along their junction-planes is noted, and the consequent 

 decomposition of such rocks, which, if subsequently compressed in 

 deep earth-movements, might present appearances considered to be 

 characteristic of " regional metamorphism." Hence the author 

 remarks that sometimes the further inference is drawn — " that the 

 Archaean schists, &c, may be only instances on a grander scale of 

 such transitional development." 



III. Metataxis. 1. Cleavage is first considered, with critical 

 observations on the theories of Mr. Harker and Mr. O. Fisher ; 

 with the latter our author does not agree. Lateral pressure, con- 

 tinued after folding and faulting have been effected, is the ac- 

 knowledged cause of cleavage, but the author does not regard the 

 accessory crystalline minerals on the cleavage-planes of many slates 

 as the result of paramorphic changes induced by the pressure. 



2. Crumpling and gnarling of the folia of some gneisses 

 and schists; and 3. The Foliation of schists are briefly noted. 

 4. Metataxic work by solar and lunar tides in a primaeval condition of 

 the globe is suggested, thus "the feeble foliation of the fundamental 



