PF.NTÏI ESKOI.A. M.-N. Kl. 



ON THE METHODS. 



Facies-petrology is a study of the relations between the chemical and 

 niineralogical composition of rocks. Attention, therefore, must be directed 

 mainly towards the in\'estigation of the constituent minerals, or the phases 

 of the rocks. 



Along with the bulk composition of a rock, its mineralogicai composi- 

 tion, or the mode, must always be known. If possible, it should be known 

 quantitatively, either estimated by geometrical methods, or by mechanical 

 anah'sis by means of heavy solutions, or calculated from the rock analyses 

 on the basis of qua!itati\-e microscopic examination. Sometimes the estima- 

 tion of the mode may be difficult or impracticable, but in any case it must 

 be hoped that no analyses of holocrystalline rocks will here- 

 after be published without at least a qualitative enumeration of 

 all the minerals. An analysis without the statement of the minerals has 

 absolutely no value for facies-petrology — and it has little \-alue for modern 

 petrology in general. 



It is also very important that the enumei'ation of the minerals should 

 refer to the same specimen as that on which the analysis has been carried 

 out. Unfortunately this is not the rule in current petrographical works, 

 and therefore the student of the laws of mineral paragenesis is often con- 

 fronted with puzzling contradictions. 



As the aim is to study mineral associations, it should be borne in 

 mind that different incompatible associations may be contained in one 

 and the same specimen, and therefore it should always be noted which 

 minerals really occur in contact with each other. For in a stable associa- 

 tion all the phases may be in immediate contact with each other. 



As analyses of the rock minerals play a dominant rôle, their separa- 

 tion for analysis is of course of the greatest importance. This is rarely 

 practicable by hand picking, in most cases heavy solutions must be used. 

 Fortunately we now possess, in Clerici's solution ^ a liquid with which 



1 E. Clerici, Preparazione di liquidi per la separazione dei minerali. Rend. R. Accad. dei 

 Lincei (Classe di scienze fis.-mat.e nat.), 1907, 187 — 195. The original paper was not 

 accessible to me, but a summary in F. Horner's dissertation „Beiträge zur Kenntniss des 

 Stauroliths", Heidelberg 1915, and in N. J. Min. 1908, II, p. 2. 



In 191 7, thallium carbonate cost, at Kahlbaum in Berlin, iio Rmk per kilogram. 



