14 ON SOME POINTS IN CHEMICAL GEOLOGY. [II. 



which is a mixture of crystallized alumina with oxide of iron, 

 have doubtless been formed. 



Waters deficient in organic matters may remove soda, lime, 

 and magnesia from sediments, and leave the granitic elements 

 intermingled with oxide of iron ; while on the other hand, by 

 the admixture of organic materials, the whole of the iron may 

 be removed from strata which will still retain the lime and soda 

 necessary for the formation of basic feldspars. The fact that 

 bicarbonate of magnesia is much more soluble than bicarbonate 

 of lime, is also to be taken into account in considering these 

 reactions. 



The study of the chemistry of mineral waters, in connection 

 with that of sedimentary rocks, shows us that the result of 

 processes continually going on in nature is to divide the silico- 

 argillaceous rocks into two great classes (mentioned on page 

 3), the one characterized by an excess of silica, by the pre- 

 dominance of potash, and by the small amounts of lime, mag- 

 nesia and soda, and represented by the granites and trachytes ; 

 while in the other class silica and potash are less abundant, 

 and soda, lime and magnesia prevail, giving rise to pyroxenes 

 and triclinic feldspars. The metamorphism and displacement 

 of such sediments may thus enable us to explain the origin of 

 the different varieties of plutonic rocks without calling to our 

 aid the ejections of the central fire. 



Mr. Babbage * has shown that the horizons or surfaces of 

 equal temperature in the earth's crust must rise and fall, as a 

 consequence of the accumulation of sediment in some parts 

 and its removal from others, producing thereby expansion and 

 contraction in the materials of the crust, and thus giving rise 

 to gradual and wide-spread vertical movements. Sir John Her- 



bauxite, serves to show an intimate relation between the origin of these two 

 bases in an xincombined state. Hydrous alumina, gibbsite, is moreover found 

 incrusting limonite, and the existence of compounds like mellite and pigotite, in 

 which alumina is united to organic acids, shows that this base may, under cer- 

 tain conditions, be set free in a soluble condition. 



On the Temple of Serapis, Proc. Geol. Soc., Vol. II. p. 73. 



