British Mineralogy. 337 



means of distinguishing the several eruptions in geological 

 chronology. 



(3) When the geological epochs of the appearance of two or 

 more intrusive or eruptive rocks are known to differ, these rocks 

 will then also be found to differ essentially in mineral consti- 

 tution. 



Unfortunately at the present time but few data are available 

 for a more extended inquiry into this interesting subject, with a 

 view to test or confirm the soundness of these conclusions. 



Although mineralogists have accumulated a large amount of 

 information regarding the crystallographic characters and che- 

 mical composition of minerals, it is but rare to find in works on 

 mineralogy any definite statements as to their mode of occur- 

 rence, mineral association, or the nature and geological age of 

 the rocks in which they may have been found imbedded. In 

 carrying out such petrological inquiries, geology must necessarily 

 go hand in hand with mineralogy and chemistry, and, whenever 

 possible, the rock-matrix of a mineral should be referred to its 

 corresponding geological epoch, so as thus to fix the age of the 

 appearance of the mineral itself. As but extremel}' - few such 

 observations are at present on record, one of the main objects in 

 thus directing attention to this most interesting and important 

 branch of mineralogical science is to induce mineralogists and 

 geologists to avail themselves of all the opportunities which may 

 present themselves, in order to collect as many observations 

 bearing upon this subject as possible, so as to serve as materials 

 for future and. more extended generalizations. 



If, now, in order to inquire into how far the present state of 

 British mineralogy can supply data for such investigations, the 

 latest published compendium of British mineralogy* be con- 

 sulted, it will at once be perceived, not only that there are but 

 few allusions to the mode of occurrence, association, or geolo- 

 gical relations of minerals, but that there is an equally remark- 

 able want of chemical analyses of many of even the most com- 

 mon British mineral species. 



According to this work, the total number of distinct British 

 mineral species amounts to 241, of which number about one-half, 

 or, more exactly, 121 have been analyzed (in this number, how- 

 ever, are included many analyses of mere varieties, in cases where 

 no chemical analyses of typical species have been made). The 

 total number of both British species and subspecies enumerated 



* Manual of the Mineralogy of Great Britain and Ireland, by Greg and 

 Lettsora. Since the publication of this work many valuable additions have 

 appeared, amongst which various papers by Haughton might specially be 

 mentioned. As yet, however, these only exist scattered in scientific perio- 

 dicals and in the Transactions of various learned societies. 



