334 Mr. D. Forbes' s Researches in 



greater number of known mineral species ; and in all parts of 

 the globe the same or very analogous minerals are, as a rule, 

 found to accompany the outbursts of similar eruptive rocks. 



Although, as before stated, the appearance of such mineral 

 compounds has been regarded as accidental and extraneous to 

 the composition of the eruptive rock-mass as a whole, a closer 

 examination leads to the conclusion, notwithstanding that they 

 often bear so relatively small a proportion to the entire mass, 

 that these minerals cannot but be intimately connected with the 

 constitution and appearance of the rocks themselves; and it 

 is anticipated that a more extended and accurate investiga- 

 tion of this subject will demonstrate, that, in like manner as 

 the occurrence of certain fossils or groups of fossils enables the 

 geological age of a sedimentary bed to be deduced, so will the 

 presence of certain minerals or classes of minerals serve as a 

 means of identifying the contemporaneous intrusions and out- 

 bursts of the eruptive rocks which at different geological epochs 

 have disturbed the earth's external crust*. 



The observations which immediately showed that the same 

 mineral might frequently occur in two or more rocks of undoubt- 

 edly different character, composition, or geological age, instead 

 of discouraging these anticipations, led, on the contrary, to a 

 careful study of the physical characters and chemical composition 

 of such minerals as were found to occur under the different 

 circumstances here alluded to, in order to discover whether some 

 characteristic differences not immediately visible to the eye might 

 not still in reality exist. 



These investigations were very tedious, and attended with 

 many difficulties, since little or no reliable information on the 

 subject was met with in works on mineralogy, and because in 

 most cases it was found necessary to visit personally the localities 

 of the minerals in question, both in order to procure authentic 

 specimens and to make sure of the penological relations of their 

 occurrence; so that, notwithstanding much time has already 

 been occupied in this inquiry, it can as yet only be regarded 

 as in its infancy. As far, however, as the investigation has 

 proceeded, the results already obtained appear most satisfactorily 

 to indicate that, whenever the same mineral is present in two 

 or more rocks of different geological age, it is usually, if not 

 invariably, characterized in each case by certain peculiarities 

 either in physical structure or chemical composition which serve to 

 distinguish it under the different circumstances of occurrence. 



It has long been known that certain species of felspar are cha- 



* The author would here specially refer to bis papers on the mineralogy 

 of Chile, in his " Researches on the Mineralogy of South America," which 

 have lately appeared in this Magazine, 



