Intelligence and Miscellaneous Articles. 315 



account the deficiency of matter in the valley of the Tyne, in the 

 hollow of Jarrow Slake, and on the sea-coast. It is also necessary 

 to obtain more precise determinations of the specific gravities of the 

 rocks about Harton Colliery than have yet been procured. Measures 

 are in progress for supplying all these deficiencies. It seems pro- 

 bable that the resulting number for the earth's density will probably 

 be diminished by these more accurate estimations. 



XLVI. Intelligence and Miscellaneous Articles. 



ON THE ARTIFICIAL PRODUCTION OF MINERALS BELONGING TO 

 THE FAMILY OF SILICATES AND ALUMINATES BY THE REAC- 

 TION OF VAPOURS UPON ROCKS. BY M. DAUBREE. 



MODERN geology admits as proved, that the rocks in contact or in 

 the neighbourhood of the massive crystalline strata have 

 undergone some modification ; to this theoretical idea they owe their 

 name of metamorpfiic rocks. 



Ever since the remarkable experiments of James Hall, an elevated 

 temperature has been admitted as the principal agent of meta- 

 morphism. Nevertheless the intervention of heat alone cannot ex- 

 plain all the details of the extraordinary modifications which rocks 

 have undergone in different countries ; complex chemical actions have 

 also evidently contributed to the alteration of the primitive type. 



In some previous researches presented to the Academy, I paid 

 attention principally to the reproduction of the minerals peculiar to 

 the stanniferous deposits and the reciprocal action of the vapours. 

 The new experiments, of which the present memoir contains the 

 results, start from the same theoretical idea, of which they extend 

 the application to a series of crystalline rocks. 



When chloride of silicium in a state of vapour and at a red heat 

 acts upon the bases which enter into the composition of rocks, it is 

 decomposed, forming chloride of calcium and silicic acid. Some- 

 times this acid remains free, sometimes it combines with the excess 

 of the base and forms simple or complex silicates. The most remark- 

 able point connected with this reaction in a chemical, and especially 

 in a geological point of view, is that the silicic acid thus formed, and 

 the silicates produced from it, have an extreme tendency to crystal- 

 lization. The crystals are small, but generally very distinct. It 

 must be observed, moreover, that the crystallization of these com- 

 pounds takes place at a temperature far below their point of fusion. 



With lime, magnesia, alumina and glucina, crystallized quartz is 

 obtained in the ordinary form of the pyramidal hexagonal prism, and 

 a portion of the base passes into the state of silicate. In this manner 

 the silicate of lime called Wollastonitc is obtained in the form of 

 rhombic tables, with two broad bevelled faces replacing the obtuse 

 tagles, which is the usual form of the native crystals. These tables 



