# A Treatise on Geology/Preface

PREFACE.
IN revising this Edition, I have employed, as the basis of classification for successive geological periods, those great natural associations of the forms of life which are expressed by the terms Palæozoic, Mesozoic, and Cainozoic. In subdividing the great classes of strata which were formed in these successive "life-periods," no material change has been found necessary, except in regard to the Palæozoic formations. Among these a much improved distribution has been effected, by labours commenced in Britain, in 1831, by Murchison and Sedgwick, and since extended by these and other geologists over great part of the globe. If some of the questions which these researches have raised are as yet only provisionally answered, they are placed in a form which may probably lead to sound and comprehensive solutions. (Vol. I., Palæozoic Strata.)
Among the many objects of geological inquiry which have been successfully prosecuted of late years, the following must be distinguished:─
The beautiful structure of slate has acquired ​more accurate expression, as a problem, by some laws of phenomena which my own observations, and those of Mr. Sharpe, have added to the well known determinations of Sedgwick; and Mr. Hopkins, ever ready to give a mathematical form to geological truth, has shown how mechanical theory desires to look at this problem. (Vol. II. p. 111, et seq.)
The dispersion of detritus from the mountains of Scandinavia, Britain, and the Alps has undergone critical re-examination, with reference to the change of level of the land, action of great waves of translation, variation of climate, extension of glaciers, and transport of icebergs. The separate labours of Murchison, Hopkins, Agassiz, J. Forbes, Darwin, and others, have thrown on this complicated subject new, varied, and effective illumination. (Vol. II. p. 11, et seq.)
I must not be prevented by the share which I have had in the Memoirs of the Geological Survey of Great Britain, from referring to this publication, and the Maps which it illustrates, for some valuable additions which have been made to British and Irish geology. Under the guidance of De la Beche, Mr. Ramsay and my other friends have cleared away from Wales whatever obscurity remained, after the strong light thrown on it for so many years by Sedgwick. Professor E. Forbes, now in charge of the Palæontological department of this Survey, has brought to the philosophical study of its rich collections matured views on the distribution of marine life views founded on assiduous dredging round the British shores, and an admirable survey of the depths of the Ægean. (Vol. I. p. 326─335.)
​Of special discoveries, geology and palæontology are always prolific. Perhaps no more striking warning of the changes which these may yet demand in geological hypothesis can be given than Mantell's unexpected Lacertian from the old red sandstone of Scotland. (Vol. I, Palæozoic Strata.)
Since the former publication of these volumes, the favour of sovereigns has been extended to three of the most loved and honoured names in English geology. We have not found it possible to alter, in all our stereotyped pages, their social designations, but this can never be misinterpreted, Geology has but one example each of De la Beche, Lyell, and Murchison.
St. Mary's Lodge, York, 1852.