M. Fonrnei's Besearches on the Geolojy of tlie Alps, 97 



According to various analyses, this rock is always formed 

 of from 59 to 70 per cent, silica, from 10 to 23 per cent, of 

 alumina, from 6 to 11 per cent, of oxide of iron, and from 2 

 to 6 per cent, of water ; frequently other bases are added to 

 this composition. " It will be observed from the first," says 

 M. Fournet, " that their composition (that of the clay-slates) 

 is homogeneous enough to entitle us to expect to find results 

 sufficiently agreeing between the metamorphism of diiTerent 

 localities, and thence also the character of generality which 

 these phenomena present."" 



The clay-slates are formed, according to the analyses, of 

 hydrosilicates, combined with silicates ; and M. Fournet 

 thinks that they owe their origin to clays, which have been 

 converted into clay-slate by calorific effects. In order to 

 prove this, he brings forward different modes of reasoning, 

 one of the most remarkable of which is drawn from the state 

 of the colouring matter of the rocks of the Alps. 



This colouring matter is commonly bitumen, which is found 

 decomposed into nearly pure carbon. Indeed, it appears, 

 from numerous examples that, among the Alps, at the con- 

 tact of the eruptive veins, the bitumen has been often con- 

 verted into graphite. He has found plates of it in the 

 saccharoidal limestone, associated with the idocrases of the 

 Coupe de Martigny. This fact had already been announced 

 by M. Elie de Beaumont, in his work on the Col du Char- 

 donnet. 



But beyond the point of immediate contact, to a distance 

 of a mile or more from the plutonic masses, there may be, 

 in certain cases, a more or less complete conversion of the 

 bitumen into anthracite ; and at a distance of several miles, 

 the bitumen is not decomposed. These distances, it will be 

 understood, may vary according to the conducting power 

 of the rocks, the power of the eruptive mass, the number 

 of veins, and, finally, the disposition of the beds. It will 

 be observed, moreover, that the above measures have been 

 taken horizontally, so that they give merely an approxima- 

 tion. However, the result is not less certain, that the whole 

 thickness of the Jurassic formation of the interior parts of 



VOL. XLIII. NO. LXXXV.— JULY 1847. O 



