The papers are of various dates; but in revising them for publication I have endeavoured, without materially changing their original form, to bring them up to the present time, and to state any corrections or changes of view that have commended themselves to me in the meantime. Such changes or modifications of view must of necessity occur to every geological worker. Sometimes, after long digging and hammering in some bed rich in fossils, and carrying home a bag laden with treasures, one has returned to the spot, and turned over the _débris_ of previous excavation, with the result of finding something rare and valuable, before overlooked. Or, in carefully trimming and chiselling out the matrix of a new fossil, so as to uncover all its parts, unexpected and novel features may develop themselves. Thus, if we were right or partially right before, our new experience may still enable us to enlarge our views or to correct some misapprehensions. In that spirit I have endeavoured to revise these papers, and while I have been able to add confirmations of views long ago expressed, have been willing to accept corrections and modifications based on later discoveries. In the somewhat extended span of work which has been allotted to me, I have made it my object to discover new facts, and to this end have spared no expenditure of time and labour; but I have felt that the results of discoveries in the works of God should not be confined to a coterie, but should be made public for the benefit of all. Hence I have gladly embraced any opportunities to popularise my results, whether in lectures, articles, popular books, or in the instruction of students, and this in a manner to give accurate knowledge, and perhaps to attract the attention of fellow-workers to points which they might overlook if presented merely in dry and technical papers. These objects I have in view in connection with the present collection of papers, and also the fact that my own pilgrimage is approaching its close, and that I desire to aid others who may chance to traverse the ground I have passed over, or who may be preparing to pass beyond the point I have reached. To a naturalist of seventy years the greater part of life lies in the past, and in revising these papers I have necessarily had my thoughts directed to the memory of friends, teachers, guides, and companions in labour, who have passed away. I have therefore, as a slight token of loving and grateful remembrance dedicated these papers to the memory of men I have known and loved, and who, I feel, would sympathise with me in spirit, in the attempt, however feeble, to direct attention to the variety and majesty of those great works of the Creator which they themselves delighted to study.