98 M. Foumet's Besearches on the Geology of the Alps. 



the Alps, has felt the influence of a temperature capable of 

 altering the bitumens. 



"We shall not follow our author in his examination of the 

 properties of slates, such as their colour, their state of mole- 

 cular aggregation, &c., with the view of finding in them the 

 products of a more or less elevated temperature. "We may 

 merely mention, that he compares two analyses, one of a clay 

 slate, the other of a petrosilex ; the first made by M. Regnault, 

 the other by Saussure ; and that he has found them very 

 nearly identical. The petrosilex may, therefore, proceed 

 from the slate ; but the word petrosilex does not well indi- 

 cate the rock M. Fournet had in view, and he proposes to 

 give to this class of rocks the name of Thermantide^ a deno- 

 mination used by Haiiy in a more restricted sense, to indi- 

 cate rocks altered by non-volcanic fires. 



The kaolinic disorganisation which the slates are liable to 

 undergo, like all the plutonic rocks, is still another charac- 

 ter which, in the opinion of our author, has been left upon 

 them by the heat to which they have been exposed. 



M. Fournet thinks that, in certain cases, the mica is a sign 

 of metamorphism, for the denomination mica does not apply to 

 a definite body ; it expresses an equivocal physiognomy, and not 

 settled features ; and this body must be formed at the expense 

 of the clay-slate, and by associating the latter with the sub- 

 stances brought by the veins. 



It is nearly the same with the talcs and chlorites which, 

 in the Alps, form complex and confused rocks by associating 

 with other minerals, as well in respect to their composition 

 as in reference to their situation and origin. 



On this occasion, the author reverts to some considerations 

 long previously established by him, relative to some termino- 

 logical errors which have been introduced in reference to the 

 crystalline slates of the Alps. They are almost always 

 spoken of as talc schists, a name which would lead us to be- 

 lieve in the great superabundance of magnesia, while in this 

 locality they are nothing else than what they are everywhere 

 else, rocks essentially aluminous. 



With regard to the modification of the limestone ; its 

 change into saccharoidal marble by the action of a high tern- 



