,df Outlines of Geology, 



oxygen ; but no application of this subject to geology renders it 

 necessary to dwell upon it, however interesting as a branch of 

 chemical inquiry, and as shewing how great a proportion oxygen 

 constitutes of the solid, as well as of the fluid, matter of our 

 globe. 



Carbonate of Limb is a very predominant ingredient in rocks 

 —marble, chalk, oolite, freestone, and all the purer varieties of 

 limestone, are essentially composed of it, and these substances 

 form strata of prodigious extent. It predominates in all the 

 varieties of shell, coral, and madrepore : with other substances, 

 but especially silica and alumina, and a little oxide of iron, it is 

 found in lias, and several marls and clays. Calcareous spar and 

 statuary marble may be selected as this substance in a very pure 

 state. The former, when transparent, is highly doubly refrac- 

 tive, as we see in Iceland spar. Its crystalline forms are very 

 various, but they all result from a piimary rhomboid^ the angles of 

 which are 105° 5' and 74° 5'. Exposed under ordinary circum- 

 stances to heat, it loses carbonic acid, and leaves quick lime, and 

 by such an experiment its composition is shewn to be 28 lime + 

 22 carbonic acid, or per cent, 56 and 44 : but if it be exposed to 

 heat and pressure, so as to prevent all escape of gaseous matter, 

 it then fuses, and retains its carbonic acid ; a fact of no small 

 importance as connected with certain theoretic considerations of 

 the Huttonian school of geology. Carbonate of lime is easily 

 recognised by its softness, and by the effervescence which it 

 produces when a little dilute muriatic acid is dropped upon it ; 

 and as none of the other carbonates constitute mineral masses, 

 this criterion is alone sufficient to distinguish it under such and 

 several other circumstances. Or, the presence of lime in solu- 

 tion may be detected by oxalate and by carbonate of ammonia, 

 which, added to muriate of lime, occasion precipitates of oxalate 

 and of carbonate of lime ; both these yield quick-lime upon expo- 

 sure to a sufficient red heat. 



Alumina rarely occurs pure in nature, but it is very abundant 

 in conjunction with other earths, and is present in all clays and 

 Tplastic earthy compounds, on which it confers the property of 



