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Edinburgh Past and Present


OUTLINE OF T H E GEOLOGY O F E D I N B U R G H AND ITS NEIGHBOURHOOD. BY PROFESSOR GEIKIE, LLD., F.R.S. FEW parts of the British Islands have been more frequently the subjects of geglogical description than the district of which. Edinburgh forms the centre. From the time when its igneous rocks were studied by Hutton, and made to bear their testimony to the Vulcanist theory of the earth, down to the present day, books, memoirs, and notices have appeared in a continuous stream, until it might be thought that hardly anything more can remain to be said or written on the subject. And yet, as each year passes, new glimpses are opened up into aspects of geology which our fathers never dreamt of, and doubtlqss, after all the living geologists and writers have passed away, succeeding generations will find the rocks and their story still inexhaustible. But though countless points of detail remain to be worked out, the general outline of the geological history of this region can now be'traced with tolerable precision: Such an outline, in language intelligible to the nonscientific reader, is all that can be attempted, or indeed seems desirable,' in these pages. Geological history is at the best confessedly imperfect, even when based upon the evidence drawn from the study of a whole country, or an entire continent, or of the globe itself. Still more fragmentary must it be when it relates only to a limited region. Under the most favourable circumstances it may lack most or all of the introductory chapters; a few scattered pages, as it were, may be all that relate the events of one of the longest and most momentous geological periods ; the narrative will suddenly break off in the middle of an interesting epoch, and when it resumes again we find that it deals with a totally different and far more recent series of events. From the T
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146 RDINBURGH PAST AND PRESENT. very nature of the materials on which the history is founded, imperfection of this kind cannot but prevail. These materials consist mainly of the sand, mud, and gravel of ancient sea-floors, lake-bottoms, and river-beds, which have been buried under later accumulations, and have been subsequently elevated into land. Seldom did these sediments enclose a tolerably complete record of the plants and animals and the physical changes of the time. Seldom, too, have they remained without suffering much from the forces which have broken them up from below and worn them away above. It is the task of the geologist to put together the fragmentary evidence to the best of his ability, not attempting to supply blanks where he can find no information, but leaving them to be filled in, if possible, from subsequent research. He is in the habit of arranging the order of events in the past history of the earth under three great divisions of geological time-(r.) Primary or Pakozoic; (2:) Secondary or Mesozoic j (3.) Tertiary or Cainozoic; to which latter a subsidiary section, (4.) Post-Tertiary or Recent, is appended. The fragmentary nature of the materials for unravelling the geological history of the.district around the Scottish metropolis may be inferred from the fact that they relate only to some of the later parts of the first, and (with trifling fragments of the third) to some of the more marked events in the fourth of these sections. The Engraving fronting the first page of this paper shows the order and relation of the rocks of the neighbourhood of Edinburgh, as they would appear if we could form a huge pillar of them, having the oldest at the bottom and the youngest at the top. Let us throw the scattered facts into the form of a narrative, and trace as succinctly as may be the successive changes of which this part of the country has been the scene. ’ The first picture we can draw is that of the bottom of a sea which seems to have stretched not bnly over the site of Britain, but over most of Europe, and to have covered alqo much of what is now land in both hemispheres. The sand, mud, and gravel deposits of that ancient ocean, laid down continuously above each other, layer upon layer, to a depth of many thousand feet, form the materials of which the pastoral hills of the south of Scotland and the mountains and glens of the Highlands are mainly composed, When the visitor to Edinburgh looks south-eastward, he sees as the boundary of the landscape, the blue outline of the chain of heights which stretch from Lammermuir into Peeblesshire. These distant bounding hills are portions of the hardened and upraised sediments of this early sea-bottom. Wandering among them, we observe how the bare rock come to the surface. We split them open,
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