204 



Each succeeding year places in a stronger point of view the 

 importance of organic remains, when we attempt to trace the va- 

 rious periods and revolutions in the history of the globe. Crystal- 

 line rocks are found associated with the strata of almost every 

 age; and the constant laws of combination which have produced 

 a certain mineral form in rocks of one era, may produce it again 

 in another. Nearly all the modifications of structure in rocks called 

 primary are also found in secondary formations: and among tertiary 

 deposits we sometimes find millstone-grit, red marl with fibrous 

 gypsum, red conglomerates, compact, subcrystalline, and oolitic 

 limestone ; in short, all the distinguishing characters of secondary 

 formations. The great barriers, which the fancy or ingenuity of 

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

 ductions of successive 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, by 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, num- 

 mulites are found only 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 ail 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 zig-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 



