21 4 Mr, De la Beche o?i the Differences 



differences in the mineralogical structure of the same forma- 

 tions. Such being the case, what still greater changes may 

 we not expect in far distant countries ? 



New facts frequently lead to new opinions ; and many of the 

 latter, which were excellent in their time, and greatly tended 

 to the advancement of geology, must be modified, should the 

 former require it. Truth should be our only object. We search, 

 in order to comprehend the structure of our planet's crust; but 

 how can we expect to accomplish this, if we imagine that Geo- 

 logy in its infancy has attained maturity ? 



A change of opinions respecting the value attributable to 

 minei'alogical structure, by no means detracts from the merit 

 of those who have been accustomed so strongly to insist on its 

 importance. On the contrar}'^, if districts have been well de- 

 scribed with reference to this character, — as is, for instance, the 

 Tarentaise by M. Brochant, — what difference does it make in 

 the merit of such a description, whether the limestones there 

 noticed be transition or lias, so long as we know, from a ge- 

 neral examination of the Alps, to which formation they really 

 should be referi-ed ? The mineralogical detail still retains its 

 original value. Without the labours of the many excellent ob- 

 servers who have attached so much importance to the mineral 

 character of formations, Geology could never have occupied 

 the rank it now does among the sciences : these labours were 

 as necessary to its development, as those of the present day are 

 to clearer and more extended views. We can only reason from 

 the facts in our possession; therefore those who come after us 

 must have much more facility in arriving at just conclusions 

 than we can ever expect to obtain. Werner is not the less 

 entitled to our thanks, though his ideas respecting the forma- 

 tion of rocks so little accord with those now most commonly 

 received ; and he is not the less, on this account, the cause of 

 a great advancement in the science. 



The necessary limits of a note of this nature preclude any 

 long detail. I shall therefore content myself for the present, 

 with a few striking examples of the very great changes ob- 

 servable in the mineralogical structure of the oolite formation 

 (including the lias) in the Alps and Italy, which it is hoped 

 will be sufficient to show the very little importance of this cha- 

 racter, when we may be desirous of determining the geologi- 

 cal epoch of a rock, and are unassisted by organic remains*. 



Those accustomed to the oolite formation, as it occurs in 



England 



* Kvcn when we have this assistance, it would seem safer, particularly 

 in the case of the more niodcrn rocks, to judge from tiie general nature of 

 these remains, rather than from any particular species supposed to be cha- 

 racteristic. 



