139 



On the Paragenetic Relations of Minerals. 



(Continued from vol. lv., page 352.) 



Lode Formations. 



By the term lode formation is to be understood a small 

 group of minerals usually associated together in lode fissures* 

 and presenting distinctive characteristics in their mode of 

 association. At the same time it must be added, that groups 

 of minerals can only be regarded as constituting any one 

 formation so long as the succession of the individual mine- 

 rals in what is termed the lode structure, or banded arrange- 

 ment of the minerals, remains the same. Repetitions or suc- 

 cessive generations of a formation likewise occur. 



There is considerable difficulty in determining and distin- 

 guishing the lode formations : thus, 1. Certain minerals oc- 

 cur in different formations, and some one mineral, especially 

 quartz, is often repeated, without ^the remaining members 

 of the group. 2. Some minerals, as iron pyrites, some va- 

 rieties of calcite, and copper pyrites, occur in so many for- 

 mations, that they cannot be regarded as distinctive. Still 

 formations are sometimes characterized by the quartz and 

 the particular abundance of pyrites. 3. In some instances 

 druses are very rare in lodes, and it is only in them that the 

 structure and succession of the minerals can be recognised. 

 4. Sometimes there are two or three formations in one lode, 

 and then it is not always easy to determine whether a mi- 

 neral belongs to one or the other. 



It appears that the minerals which serve best to distin- 

 guish the lode formations are either some of those siliceous 

 species which are not products of decomposition, or some of 

 the true ores. There is still a want of some kind of scientific 

 nomenclature for these phenomena, but the paragenetic rela- 

 tions are perhaps too little understood, and the relative dates 

 of lodes too little known, to warrant the adoption of one as yet. 

 Nevertheless the paragenetic grouping of a few but constant 

 minerals in lodes is too evident to escape notice. Such, for in- 

 stance, is the case with minerals containing cobalt, nickel, bis- 

 muth, and arsenic ; lead and zinc ; tin and scheel, and the very 



