PREFACE TO THE FOURTH EDITION. 



If the same conchologists were interrogated respecting the power 

 which the ancient inhabitant of the shell naight possess of changing 

 its structure when placed in different circumstances, they would be 

 compelled to confess their ignorance. A knowledge of fossil shells 

 is highly useful to the geologist in cases where the superposition of 

 strata cannot be ascertained ; but fossil shells alone, give us less pos- 

 itive information respecting the ancient condition of the globe, than 

 the organic remains of other classes of animals, or of vegetables 5 

 because, for any thing we know to the contrary, all the species of 

 molluscous animals that inhabited these shells, may have been capa^ 

 ble of living in the same medium, and under the same conditions. 

 But different species of vertebrated animals, and plants, must have 

 existed under very different conditions, on land or in water. M. 

 Boue, an enlightened and indefatigable continental geologist, to whoso 

 labours the science is greatly indebted, is meritoriously endeavour- 

 ing to resist the absurd attempt, to force Fossil Conchology into the 

 chair of Geology. I trust his example will be followed by English 

 geologists. Indeed, I am convinced that many of the frivolous dis- 

 tinctions introduced by conchologists will soon pass away, as those of 

 mineralogy have already passed and that these two branches of 

 natural history, will take their proper stations as auxilliaries subser- 

 vient to geology. 



It will be seen, by the tides to the new chapters in the present 

 volume, that they comprise various subjects connected with impor- 

 tant enquires relating to the Theory of the Earth. The opinions of 

 the author have not been rashly advanced, to oppose or maintain the 

 systems of other geologists : they are the result of long-continued re- 

 flection, on what appeared to him the most probable explanations of 

 geological phenomena. The author says probable^ because he con- 

 siders that the words truth and certainty cannot yet be introduced 

 with advantage into geological theories. 



The author requests the experienced geologist, who may honour 

 this volume with the perusal, to refer to Chap. XXII., in Vv^hich he 

 will find that the doctrine of the elevation of mountain ranges, at dif- 

 ferent epochs, was distinctly announced, and was published by him 

 in the year 1823, supported by the same principles, as those recent- 

 ly advanced by M. Elie de Beaumont. He has farther proved^ that 

 the elevation of large islands, and continents, was long posterior to 

 the elevation of mountain ranges. 



* See the end of the Preface to the first edition. Some of the distinctions in 

 Mineralogy, on which most important conclusions have been founded respect- 

 ing the formations of rocks, are now known to be erroneous : magnesian minerals 

 were all stated to be of aqueous origin. Pyroxene (Augite) was considered as an 

 unerring criterion of igneous products, and to be an entirely distinct species from 

 Amphibole (Flornblende) : they are now proved to be identical minerals, convert- 

 ible into each other, according to the degree of temperature under which they are 

 crystallized. Observations on the true value of Fossil Conchology will be found 

 inChapter XVII. 



