8 GEOLOGICAL MEMOIRS. 



It appears far more natural to admit that the elements of the meta- 

 morphic rocks already existed in them, and have not been introduced. 

 Certain sedimentary dolomites and limestones have evidently under- 

 gone a fusion and subsequent crystallization (St. Gothard). Chiasto- 

 lite, Felspathic-granites [ ? ], and many other silicates may have at any 

 time been produced by the intense heating of sedimentary rocks in 

 which their constituent elements were already present. Felspar cry- 

 stals originate where the sedimentary rock contains the aluminous 

 alkali-silicates of the pyrogenous rocks from the destruction of which 

 it has been formed. 



The author indicates a new origin for felspar by the wet method. 

 This soluble combination, which is formed in the laboratory by the 

 precipitation of alumina with the natron-silicates, occurs also in the 

 clays, in spite of the solubility of the soda, and must have necessarily 

 been precipitated in the earliest times from all the ancient seas so 

 rich in kali- and natron-silicates. The presence of these alkali-sili- 

 cates has given rise to the immense quantity of quarzites, jaspers, 

 and flint-rocks, which are so abundant in the oldest formations. 



These quarzites, jaspers, felspars, &c. have been precipitated che- 

 mically with all sediments, and their predominance in certain places 

 of the sedimentary rocks has rendered them so homogeneous and 

 hard, that recourse has been had to the hypothesis of an extensive 

 metamorphic felspathization : — as for instance in the case of the 

 greywacke of the Vosges. [T. R. J.] 



On Childrenite*. By C. Rammelsberg. 



[Poggendorf's Annal. Ixxxv. p. 435, &c. ; and Leonh. u. Bronn's N. Jahrb. f. Min. 



u. s. w. 1854, p. 322.] 



This mineral occurs in a vein in the George and Charlotte Mine near 



Tavistock in Devonshire, with spathose iron, quartz, and copper-pyrites. 



It is also stated to have been found near Callington in Cumberland. 



Form, rhombic octohedron ; the side angles, according to Brooke, 



= 97° 50', the sharper terminal angles =120° 30' [?102°30'], the 



blunter 130° 20'. Combinations, several. Colour, yellowish and 



blackish brown, and blackish. Transparent. Lustre, brightly vitreous. 



Hardness =5. Powder, yellowish. Specific gravity, from 3*247 



to 3'28. Composition: — 



Phosphoric acid 28*92 



Alumina 14'44 



Oxydulated iron 30*68 



Oxydulated manganese 9*07 



Magnesia 0*14 



Water 16*98 



100*23 



Formula :— (2RT+ AFP) + 1 5H. [T. R. J.] 



* See also Brooke, Quart. Journal of Science, vol. xvi. p. 274 ; and Brooke's and 

 Miller's Mineralogy, p. 521. — Transl. 



