30 Dr. Mac CuUocli on the Qiigin^ Materials, 



the repository for the facts of this description which do not ap- 

 pertain to the present paper. 



The earths, which enter into the composition of those minerals 

 that form the ordinary or essential ingredients of rocks, are 

 silica, alumina, lime, and magnesia. If the other earths are oc- 

 casionally found, it is rather in those minerals which cannot be 

 considered essential to the constitution of rocks, but which are 

 frequently imbedded in them. To these earths must be added, 

 iron in different states of oxidation ; and, from some observa- 

 tions which I have made on quartz rock, limestones, and traps, 

 in that of a carbonate also. Potash and soda are, lastly, essential 

 ingredients in some rocks, and it remains to be proved whether 

 lithia may not sometimes be present where one or other of these 

 has been suspected. As the earths, as well as the alkalies, are 

 now known to be oxides, and as it is also known that silica, at 

 least, acts the part of an acid in some mineral combinations, it 

 is probable that we have much yet to learn, respecting the origin 

 and formation of many rocks ; but whatever splendid probabilities 

 may open on us from this new source of knowledge, we are 

 scarcely yet able to build any rational conjectures on it. 



The simple minerals formed of these substances, and which 

 constitute the essential ingredients of all rocks, are quartz, fel- 

 spar, mica, hornblende, hypersthene, diallage, augit, serpentine, 

 compact felspar, actinolite, chlorite, talc, and schorl. Some of 

 these are, however, far more abundant than others ; nor is it 

 easy to define the limit between them and those which may be 

 considered accidental; which are occasionally imbedded in rocks, 

 as their natural repositories. But it is unnecessary to dilate on a 

 subject which is sufficiently detailed in the work above-mentioned. 

 It is sufficient to quote, as examples, garnet, which is sometimes 

 abundant in micaceous schist, or sparingly dispersed, or altoge- 

 ther absent, without affecting its essential characters ; and spo- 

 dumene or corundum, which may thus exist in granite. 



If we consider the great number of minerals, thus generally 

 distinguished into essential and unessential, which nature has 

 formed, or if even we limit our views to those which may be 



