478 Beviews — The Survey Memoir on the Scottish Uplands. 



of its apparently simple tectonic structure. Two examples must 

 here suffice. The new reading of the succession interpi'ets the widest 

 visible 'synclinal' of the Leadhills as its broadest anticlinal form, 

 and regards the * anticlinal ' of Dumfries and Hawick — the very 

 backbone of the Uplands — merely as a squeezed-in synclinal of no 

 stratigraphical importance whatsoever. 



Nor would the theoretical picture of the physical conditions of the 

 Upland region in Silurian times, drawn by any student of the zonal 

 methods, bear much resemblance to that limned by anyone who had 

 mapped the country on the old regional plan. 



Tlie detailed phenomena worked out by the zonal method call 

 upon us, explicitly or implicitly, to believe that in Arenig and 

 Llandeilo times where now in part the Scottish Uplands rise there 

 lay a region of broad, deep, and clear seas, over which the slowly 

 moving currents drifted floating fields of tangle and seaweed, from 

 which, perhaps, hung suspended the plant-like graptolites. Into 

 these clear seas came for ages little or no land-derived sediment, 

 save now and again the dust from a distant volcano. Their deep 

 waters were bordered to the north-west by islanded shores, at first 

 crowned by active volcanoes, but afterwards buried up with many 

 oscillations under massive sheets of pebbles, sand, and mud, often 

 marvellously prolific in animal life. Slowly the floor of the deep 

 sea became shallowed, and the shore-derived sediments travelled 

 farther and farther seaward, each formation in the earlier part of the 

 ascending series thus showing a gradual thickening in a landward 

 direction, each formation in the later -part showing an increased 

 thickening as a whole. By the close of Silurian times the sea-floor 

 had all emerged as solid land to face the wear and tear of rain and 

 rivers, having in the process of time become wrinkled and folded in 

 a most marvellous way. To-day, after eons of denudation, the old 

 rock formations of Silurian times, thick and massive in the directiou 

 of the ancient shore-lines, thinning away almost to nothing where 

 lay the deeps of the seas, still lie, each in contact, it is true, with its 

 natural predecessor and successor in time ; but so wrinkled and 

 jammed together are they, so twisted and tangled locally, that only 

 by studying each rock-sheet thread by thi'ead, as it were, where it 

 shows out now and again through the rents and the tatters of the 

 sheet now above it, can we say with certainty which of those sheets 

 it is we are studying, and what were the local circumstances of its 

 deposition. 



These are a few of perhaps the most striking contrasts between 

 the results obtained by the interpretation of the phenomena presented 

 by the convoluted rocks of the Uplands : on the one hand, by the 

 older and simple /orma^/ona/, collective, or areal methods, as followed 

 by Nicol, Sedgwick, Murchiscm. Harkness, and others, and the officers 

 of the Survey up to the year 1873 ; and on the other hand, by those 

 M'ho have used, in whole or in part, the newer and more complicated 

 zonal methods, as developed by British and foreign geologists, 

 broadly speaking, between the years 1870 and 1886. 



Before, therefore, any general memoir upon the Silurian rocks of 



