CHAPTER II. TERRACES OF GLEN ROY, AND LAKES OF AUVERGNE. TERRACES OF GLEN ROY PROVE ENORMOUS DENUDATION IN FORMER TIMES.. WHY SHOULD THIS DENUDATION HAVE CEASED NOW? LAKES OF AUVERGNE FIRST FILLED WITH ALLUVIUM, THEN CUT INTO HILLS AND VALLEYS BY RAIN AND RIVERS. THIS IS CONTRARY TO THE SUBMARINE AND WHY VALLEYS IN FAVOUR OF THE SUBAERIAL THEORY. I WILL again begin the chapter by applying Lyell's own words to himself: A false theory, it is well known, may render us blind to the facts which are opposed to our prepossessions, or may conceal from us their true import when we behold them.' In reference to Lyell's own submarine theory of forming valleys, I had intended to extract from the beginning of his Elements,' 2nd edition, to the end of his 'Principles,' 7th edition, all passages contradicting that theory in particular, or each other generally. I found that I became far too voluminous. For the questions of denudation in the formation of valleys, and of deposit in the formation of all strata, alluviums, and drift-beds are of such universal and constant occurrence and recurrence in geology, that there is scarcely a page in Lyell's works in which he will not be found an unwilling and unintentional witness against his own submarine theory, and in favour of the common-sense plan of forming valleys by rain and rivers.' But though I have thus been obliged to curtail my extracts, I have still retained my original order of operations. In reference to the formation of the parallel Terraces of terraces of Glen Roy, Lyell (Elements,' chap. vii.) 6 Glen Roy prove enormous denudation in concurs in these words with Dr. M'Culloch, former times. Sir T. D. Lauder, Mr. Darwin, and Mr. Agassiz. There is one point, however, on which all are agreed, namely, that these shelves are ancient beaches, or littoral formations, accumulated round the edges of one or more sheets of water, which once stood at the level, first of the highest shelf, and successively at the height of the two others. It is well known that, wherever a lake or fiord exists surrounded by steep mountains, subject to disintegration by frost or the action of torrents, some loose matter is washed down annually, especially during the melting of snow, and a check is given to the descent of the detritus at the point where it reaches the waters of the lake.' Now here is a denudation not longitudinal, or, as Lyell has just said, from the action of torrents,' or, as Professor Sedgwick would say, acting on lines,' but lateral. It is a denudation of an enormous breadth of hill-side. For, as Lyell says, these terraces extend over a space of many leagues in length,' and 'they are from ten to sixty feet broad.' 6 And it is not, as Lyell has said, that a check is given to the descent of the detritus at the point where it reaches the waters of the lake,' but that all denudation ceases there. Subaqueous regions are indeed the regions of deposit, as subaerial regions are the regions of denudation. But deposit takes place at the bottom, not at the top of water; and these terraces are not formed by the accumulation of deposit, but by the absence of denudation compared with the parts above them. The 6 6 detritus,' from the denudation instead of being arrested at the point where it reached the waters of the lake, went to the bottom of the lake, and has long since been cleared out of the valley by the same power which brought it in to the valley—rain. this denuda tion have ceased now? If these philosophers are all agreed (and who Why should will disagree with them?) that this enormous denudation of the hill-sides and tops took place in former days, should they not give us some reason why it should have ceased in these days? This was so printed in 1857, but since that time the once sane Lyell has gone into all the insanities of glacialism; and he now requires' ice-dams,' to turn the water back the wrong way, over the water-partings or so called cols! And since that time I have had the pleasure on the 18th September, 1863, to walk up Glen Roy, and I published the following in the Athenæum. I think that these terraces are not the remains of shores of any kind, either sea-shores or lake-shores. They are the remains of patches of alluvial plains.' Years ago (Rain and Rivers,' page 4 of this edition), I described at length the birth, life, and death of these transitory alluvial plains. But I was not till now aware of their disposition to set up their own gravestones. Shortly, these alluvial patches come and go thus. Every barrier of hard rock which crosses the bed of a river or valley becomes a negative key to the depth of the river and valley above the barrier. Thus deep shalt thou go and no deeper. But as denudation is ever at work, the bed of the river and of the valley above the barrier become horizontal at the same level as the barrier. The flood waters are checked at the barrier, overflow the horizontal plain, and form an alluvial patch. The barrier sinks from erosion. The alluvial patch is no longer overflowed. The river is confined between walls of its own building. It sets to work to tear them down. It carries off the alluvial patch which it had deposited till it leaves only two terraces against the hill-sides. Having cleared a horizontal plain below the level of these terraces and at the new level of the barrier, the river again floods over and begins a new alluvial patch below the two parallel terraces. We only want eyes to see that parallel terraces are being formed in this way, at this instant, along the beds of Glen Roy and Glen Spean, and these recent terraces are composed of the same materials as the ancient ones. In Lyell's elements two of these lower terraces by the river side are actually delineated. But I was thoroughly astonished at the multiplicity of terraces with which the hillsides are scored. From Sir T. Dick Lauder's drawing, and Mr. Jamieson's map given in Lyell's |