Geological Society. 307 



geologists has at different times set up between the mineral pro- 

 ductions of succe?sive periods, have been thrown down, one after 

 the other. I do not deny the importance of mineralogical charac- 

 ters ; I only mean to assert that, taken by themselves, they are no 

 certain indications of the age of any deposit whatsoever. 



In reasoning from organic remains, from the succession of large 

 groups alone can we establish any safe induction. Positive rules 

 founded on the presence of particular genera or species are of com- 

 paratively small value. But the mind becomes wearied and bewil- 

 dered by the endless succession of individual forms, and delights to 

 take refuge in some generalization : and generalizations would be 

 excellent things if we could be persuaded to part with them as easily 

 as we form them. They might then be used like the shifting hypo- 

 theses in certain operations of exact science, by help of which we 

 gradually approximate nearer and nearer to the truth. 



In England, and many other parts of the north of Europe, nura- 

 mulites are only found in tertiary rocks, and orthoceratites only in 

 those of the transition periods ; but in the secondary limestone of 

 the Alps we find, abundantly, both orthoceratites and nummulites. 

 Ammonites and belemnites have not yet been found among the 

 strata called tertiary. But should the chasm between the secon- 

 dary and tertiary systems ever be filled up, it may be as difficult 

 to draw any line between them, as it now is to draw the line be- 

 tween the transition and secondary series. Belemnites descend no 

 lower than the lias. Ammonites descend among the transition rocks: 

 and it has been remarked, that in all the deposits under the lias, 

 the concamerationS of this genus are of a simpler figure (being 

 marked at their junction with the outer shell only by lines undula- 

 ting or in rig-zag, ) than those of the corresponding fossils in the 

 higher formations. As far as regards the English carboniferous and 

 transition series, this rule is true. But the only ammonite I ever 

 found in the magnesian limestone had those suture-like markings 

 which distinguish this genus in the upper secondary beds. The 

 producta is not found above the magnesian limestone (zechstein): 

 it occurs abundantly in the lower part of that formation, and it 

 also abounds among the fossils of the transition periods. Cer- 

 tain plants are eminently characteristic of our coal formations; but 

 in England they also occur in the sandstone beds, which alternate 

 with the mountain limestone. Near Magdeburg they are found in 

 grauwacke; and M. Elie de Beaumont has, on the south flank of the 

 Alps, found the same vegetable forms in beds of the age of our lias. 

 Positive and negative rules like these, when kept in subordination 

 to new facts, are of the greatest value ; for they record in a few 

 words the result of many observations. 



When wo examine a scries of formations which are in contact, 

 we constantly find them passing into each other : and when we 

 place tlie groups of fossils derived from the successive terms of 

 the series in the order of superposition, their passage is still more 

 striking. I do not mean by this to vindicate the transmutation of 

 species ; because that doctrine is opposed by all the facts of any 



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