Januaey 1, 1897.] 



KNOWLEDGE. 



15 



Elements of Differential Calculus, By Edgar W.Bass. (Chapman 

 & Hall.) 17s. net. 



Notes for Chemical Students. By Peter T. Austen, Ph.D. 

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Diagrams of Terrestrial and Astronomical Objects and Phenomena. 

 Bj K. A. Gregory, F.E.A.S. (Chapman & HaU.) 21s. net. 



Whitaler's Almanack; 1897. (Whitaker.) 2s. 6d. 



Light as the Interpenefration of the Law of Graviti/. By 

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Confidences of an Amateur Gardener. By A. 11. Ilewsmith. 

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Charles Danciii and the Theory of Natural Selection. By 

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Smithsonian Contributions to Knowledge. (Wesley & Son.) — " Life 

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London University Guide and University Correspondence Calendar, 

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Hygiene for Beginners. By Ernest Septimus Reynolds, M.D. 

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The Story of Forest and Stream. By James Eodway, F.L.S. 

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A Handhool- of the Game Birds. Vol. II. By W. R. OgUyie- 

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THE AGE OF MOUNTAINS.-I. 



By Prof. J. Logan Lobley, F.G.S. 



THE marvellous foldings and plications of rocks 

 observable in mountain and hill ranges, which 

 were lately described in Knowledge, »- suggest an 

 inquiry as to when they were produced. To reply 

 to this inquiry a knowledge must be obtained of 

 the geological age of the elevations exhibiting these great 

 and wonderful phenomena. 



" The everlasting hills " is a phrase with which we are 

 aU familiar; indeed, we are so accustomed to look upon a 

 mountain as a type of all that is stable and enduring, that 

 it would to many be somewhat startling to be told that the 

 hnis and the mountains of the globe are constantly 

 changing and being worn away, and much more so to be 

 informed that, compared with the age of the oldest rocks, 

 many are but things of yesterday. But it is well known 

 to all observers of nature that every rainstorm, every 

 torrent, carries away from the summits and sides of hills 

 and mountains a large amount of solid material ; that 

 ice, and snow, and frost, and thaw, take from the peaks and 

 the higher regions of lofty mountains enormous quantities 

 of the hardest rocks ; that these are hurled down to the 

 base of the precipices, and carried to the rivers of the 

 plains. The " screes " of our own Cumberland Lake 

 District, and the broad desert of stones derived from the 

 summi's of the Alps, which in the dry season extends as 

 the wide bed of the River Po for hundreds of miles , are but 

 obnous illustrations of the rapid waste of hUls and moun- 

 tains. The great plain of Lombardy itself has been 

 formed by the filling up of a shallow sea with the debris 

 brought from its bounding mountains. It is not, however, 

 to the wearing away or destruction of mountains to which 

 attention is now invited, but to their various ages and the 

 comparatively recent elevation of many of the mountain 

 ranges of the globe. 



By the " age of a mountain,'' it must be borne in mind, 

 is not meant the age of the rocks of which it is composed, 

 but the age of the mountain as the elevation we now see 

 it, or, in other words, the geological period in which it 

 assumed that prominence and importance as a feature of 

 the earth's surface which is given to it by a considerable 

 elevation above the level of the sea. 



The maximum geological age of a mountain can be 

 approximately fixed by ascertaining the age of the most 

 recent of the rocks occurring at a considerable height 

 above its base, since ihese rocks must have been formed 

 under water at a low level, and so must be older than the 

 elevation which placed them at the level they now occupy. 

 In order to determine the geological age of a mountain it 

 is requisite, therefore, to ascertain the most recent or 

 newest strata forming any part of its summit or flanks. 

 It can then safely be said that elevation, and to that extent, 

 has taken place since the period of the deposition of those 

 recks. But it cannot be safely said that the range is of 

 later date than the geological epoch of the formation to 

 which the most recent strata belong, because the periods 

 during which the " formations" of the sedimentary rocks 

 were individually deposited were each of such long duration. 

 As was well shown by Sir Charles Lyell in reply to Mr. 

 Elie de Beaumont's arguments in support of the theory of 

 the sudden rise of mountain chains, it was quite possible 

 for a mountain range to be partly composed of Cretaceous 

 strata, and yet to have been upheaved in the Cretaceous 

 epoch. The period indicated by that phrase was of sufficient 

 duration to allow of deposition, accumulation, consolidation, 

 upheaval, and great elevation, and even of subsequent sub- 

 sidence beneath the waters of the ocean. And the same 

 might be said of the other great geological epochs of the 

 earth's history. Unless there be other evidence, therefore, 

 it can only be said that the elevation took place since the 

 commencement of the epoch of the latest rock, or subse- 

 quent to the close of the preceding period. 



There may be other evidence, however, which will enable 

 a more positive statement to be made, since we can, by a 

 knowledge of the geology of a district, even assert that an 

 elevation within its area is of later date than the close of 

 the epoch of the rock of which it is composed. An example 

 is not far to seek, and one on a small scale is in the im- 

 mediate neighbourhood of London. In the Thames Valley 

 there are several elevations rising to the dignity of hills. 

 Those which attain a certain elevation, as Hampstcad and 

 Harrow hills, are capped by sands, which cappings are 

 outliers of the Bagshot Sands, the hiUs below these sands 

 being composed of the London Clay. From this it cau be 

 asserted that the lower hUls, as the Norwood hills, although 

 not capped by Bagshot Sauds, are also newer as separate 

 elevations than the deposition of the sands of the higher 

 hills, since they have all been the result of cu-cum- 

 denudation during the same period. 



V0LC.\NIC .MOUNT.UNS. 



Volcanoes are so distinctly different from all other 

 mountains in form, structure, and origin, that they should 

 obviously be considered separately, and placed quite apart 

 in any consideration of mountain age, although they are 



-See KxowLEDGE, July. 1896. 





Eio. 1. — Prehistoric Vesuvius : a Single Cone. 



often, parasitically, as it were, situated on a mountain range. 

 In many cases these volcanoes are lofty mountains, with 

 beautifully regular conical summits. These thus attract 

 the eye even when surrounded by still greater elevations, 

 and often give much beauty and great interest to the 

 scenery from amidst which they rise. The origin and 



