Fossil Organic Remains. 47 



At present, we are unable to judge at what point this blend- 

 ing together will cease ; and hence it follows, that the real 

 species, between which no passages take place, are not yet as- 

 certained. But when the mode of determination is not yet satis- 

 factorily settled in a science (natural history, to U'hich palceontology 

 belongs), no determinations, if they are to be deemed satisfactory, 

 ought to be founded upon it in another science (that of geognosu 

 tchich takes natural history for granted, icithont belonging to it). 



3. The age of the fossils can only be ascertained from the 

 age of the mountain-masses in which they are found ; and that, 

 supposing that a proper idea be formed of the subject, can only 

 be determined from the relations of superposition of the moun- 

 tain-masses.* But when the relations of superposition of a 

 mountain-mass are sufficiently known (the nature of the rock 

 gives rise to fewer difficulties), no other assistance is requisite 

 for the determination of its age, that is of the formation to 

 which it belongs, provided the idea of a formation actually be 

 a reality, which is here supposed and assumed, because the 

 present place is not suitable for entering more deeply into the 

 subject. 



The result of these observations is, that, in the search for 

 repositories of useful minerals, we must not trust to determi- 

 nations of this kind, viz. to determinations of certain forma- 

 tions of secondary rocks by means of their petrifactions. I 

 shall, in another place, speak further of the proper and legiti- 

 mate use of petrifactions in this occupation. 



In order to prosecute successfully the search for valuable 

 mining repositories in secondary rocks, it is not enough to be 

 well acquainted with the different members of the series recog- 

 nised by geognosts, and to explore them when they are present 

 in a district ; but we must investigate each separate bed, or each 

 individual rocky mass, to whatever formation we may be in- 

 clined to refer it, and must endeavour to discover the sequence 



• Another mode of procedure for accomplishing the determination of the 

 relative age of petrifactions, and by means of it, of formations of rocks, that, 

 namely, by observing the relations of the remains of still living species to 

 those of the extinct, although it may possess value in speculative geology, 

 cannot come into ion in an occupation where we must rely on di- 



rect observation alone. 



