Scientific Intelligence — Geology. 393 



to separate more or less completely in tho decomposition of the rock. 

 Water alone is found in much greater proportion in the decomposed 

 than in the unaltered rock. The produce of the alteration in tho 

 rock tends to approach more and more near to a hydrated silicate of 

 alumina, than to a clay. These results are quite in unison with such 

 as he had formei'ly obtained, and from which he deduced the two 

 following principles, \st, In the decomposition of silicate not contain- 

 ing alumina, we constantly find that the silex, lime, and magnesia, 

 are eliminated. Sometimes, however, the iron remains in the resi- 

 duum of the decomposition in the state of peroxide, sometimes it dis- 

 appears with the other bases. 2d, In the decomposition of silicates 

 containing alumina and the alkalies, with or without other bases, the 

 alumina is concentrated in the residuum by retaining the silex and 

 fixing tho water, while the other bases are carried along with a part 

 of the silex. The final produce approaches more and more to a hy- 

 drated silicate of alumina. 



Almost all the rocks of igneous origin contain alumina, and con- 

 sequently, yield an argillaceous residuum by decomposition under the 

 influence of the atmosphere. The author endeavours to shew, in his 

 memoir, that we cannot ascribe the clay of stratified formations to 

 any other origin than the mechanical abstraction of the residua of 

 the decomposition of igneous rocks. 



Finally M. Ebelmen examines, at the conclusion of his memoir, 

 one of the most important questions relating to the natural history 

 of the globe, that of the relations which necessarily exist between 

 the phenomena of the alteration of rocks and tho composition of at- 

 mospheric air. " The difterent bases which separate from the silex 

 by the decomposition of igneous rocks determine, in fact, the precipi- 

 tation, the mineralisation of the oxygen and of the carbonic acid. 

 The last element, in particular, is absorbed in great quantity, and a 

 simple calculation shews that a small body of decomposed plutonic 

 rocks is sufficient for the complete precipitation of the carbonic acid 

 contained in the air. Now, the argillaceous beds of stratified for- 

 mations induce tho decomposition of innnense masses of plutonic 

 rocks; and, consequently, the precipitation of quantities of carbonic 

 acid out of all proportion with those actually existing in the atmos- 

 phere. This result may be explained without any necessity of ad- 

 mitting that the air has possessed, in the different geological epochs, 

 a very different coanjiosition fi-om which it presents at present. 



" I observe, in volcanic phenomena," says M. Ebelmen, " tlie 

 principal cause which restores to the atmosphere the carbonic acid 

 which the decomposition of rocks continually precipitates from it. 

 We know that this gas is disengaged in abundnnre from the ground 

 in the neiglibouihood of active volcanoes, and even from extinct vol- 

 canoes. It is interesting to witness the formation of igneous rocks, 

 accompanied witli the disengagement of a gas, which tho destructiitu 

 of tlicse same gases will precipitate. The central heat of tho globo 



