336 Mr. D. Forbes's Researches in 



Olivine also differs in chemical composition, according as it is 

 found in magnesian limestones, metamorphic schists, or volcanic 

 rocks *. 



And this list could easily be extended. Not only, however, 

 do the same minerals when occurring in rocks of different nature 

 possess distinct peculiarities which distinguish them from one 

 another, but, as has been already shown [Christiania Naturfors- 

 kere Modet Forhandlinger, 1856f), this may even be the ease 

 when they occur in rocks of the same character but of different 

 geological ages. Thus certain younger granites which are com- 

 mon in Norway, Sweden, and Russia (as well as in the United 

 States of America) are characterized not only by the appearance .of 

 numerous mineral species containing the rare metals cerium, lan- 

 thanum, yttrium, &c, but also by several of the usual minerals 

 (common to most granites) which otherwise do not contain any 

 trace of these elements being found to have a portion of their 

 ordinary bases replaced by the oxides of these metals. Thus, 

 in this younger granite, epidote is represented by orthite, titanite 

 often becomes yttrotitanite, garnets frequently contain several 

 per cent, of yttria or oxide of cerium, apatite contains crypto- 

 lite (a phosphate of cerium), and the scapolite, according to Her- 

 mann's analysis, is hyposclerite (containing some 3 per cent, of 

 oxide of cerium) . 



The results of the study of the mineral occurrence in eruptive 

 rocks, along with that of the physical characters and chemical 

 composition of these minerals, as well as a careful examination 

 of the rocks themselves in the field in many parts of the world, 

 have confirmed the author in the belief that such minerals as 

 previously had been generally regarded as extraneous or acci- 

 dental are in reality truly characteristic of the rocks themselves, 

 and that even such common species as are usually recognized as 

 rock- constituents also possess, when they occur in rocks differing 

 widely in character and geological age, peculiarities either of 

 physical structure or chemical composition which may serve to 

 distinguish them from one another. This inquiry also led him 

 to conclusions already made public, that :— 



(1) Eruptive or intrusive rocks of identical mineral constitution 

 have made their appearance or intrusion into the earth's crust at 

 similar geological epochs. 



(2) The minerals, or classes of minerals, accompanying or as- 

 sociated with such intrusive or eruptive rocks may serve as a 



* Carbonate of lime is generally admitted to indicate differences in the 

 origin of a rock in which it occurs, according as it is present in the form of 

 calcite or Arragonite; and this is doubtless also the case with sulphate of 

 lime when occurring respectively as gypsum or anhydrite. 



\ An abstract in the Quarterly Journal of the Geological Society for 

 August 1853. 



