PREFACE TO THE FOURTH EDITION. 



The present Volume contains above one fourth more letterpress 

 than the Third Edition : being printed closer, and in a fuller page, 

 in order to comprise numerous additional facts, and the important 

 discoveries recently made in Geology. There are five entirely new 

 chapters, beside considerable additions to most of the former chap- 

 ters. The new chapters in this Edition are : — Chap. XV. On the 

 Formation of Secondary Limestone and Sandstone, and on the pro- 

 gressive Development of Organic Life. Chap. XVII. On the Qua- 

 ternary Strata. Chap. XX. On Subterranean Currents, and on Cav- 

 erns. Chap. XXII. On the elevation of Mountain Ranges and Con- 

 tinents. Chap. XXIV. On the Temperature of the Earth; on Cen- 

 tral Heat; and on Astronomical Causes illustrative of Geological 

 Theories. Beside two new plates, the present Volume contains also 

 numerous wood cuts. 



Since the publication of the Third Edition, the Author has revisit- 

 ed several of the localities which were the scenes of his earliest in- 

 vestigations ; he has also examined certain parts of England, of which 

 the geology was dubious ; and has inserted in this work such altera- 

 tions as were deemed necessary. These, however, bear a small 

 proportion to the valuable labours of foreign and English geologists, 

 during the last five years, of which an account is given in different 

 parts of the volume. In a preliminary dissertation on certain living 

 species of animals that elucidate fossil conchology, and also in the 

 work itself, the author has endeavoured to direct the attention of ge- 

 ological students to a subject hitherto much neglected. Great im- 

 portance is attached to the study of fossil shells ; but the character 

 of the animals that inhabited them, or the power they might possess 

 of modifying the form of the shell under various circumstances, has 

 scarcely been thought of. Some French conchologists are endeav- 

 ouring to establish the doctrine that fossil conchology, independent of 

 the succession and stratification of rocks, is the only true basis of 

 geology ; and a trifling difference in the form of a shell, is deemed 

 sufficient to constitute a new species, and to warrant the most impor- 

 tant conclusions respecting the age of rock formations. Cato, when 

 the Roman Haruspices were gravely examining the entrails of the 

 sacred victims, to ascertain the future revolutions of empires by the 

 convolutions of the intestines, said, that he much wondered how they 

 could refrain from laughing, whenever they looked each other in the 

 face. Surely we might say the same to fossil conchologists, when they 

 gravely attempt to ascertain the past revolutions of the globe by the 

 convolutions of a shell. 



