May 20, 1880] 



NATURE 



69 



dates, and the various works which Jhey had written 

 are given here in the following table : — 



Theories. Wkiteks. 



^ c Fingal Old inhabitants. 



Works. 



Eg 



Hunting roads /Old inhabitants. 



of kings ... IPennant 



Aqueducts for ^plJ^yfJ^if 

 I irrigation ... J ' 



j-SirGeo.S.Maclcenzi 



j Prof. Rogers ... . 



769 Pennant's "Tour," 1771. 

 S16 Proc.Roy. Soc.Edin.,i8i6- 



Edin. Phil. Jour., Jan., 



184; 



Inst,, Lend., 



[■ Danvin 

 Lyell (vL, 

 Chambers 



1839 London Phil. Trans., 1839- 



25)... 1841 ■' Elements of Geology." 



1848 " Ancient Sea Margins. " 



J Rev.R.Boog Watson 1866 Geol. Soc. Lond. Jour., 

 ^ February, iS66. 



;86g Geol. Soc. Lond. Jour., 



August, 1869. 

 877 " Parallel Roads of Glen- 

 Roy," printed privately. 

 :Si7 Trans. Geol. Soc. Lend., 



1 Prof. NIcgI ... 



Campbell of Islay . 

 /Macculbch 



Lubbock 



Babbage 



Rev. T. Brown 



Agassiz (visited 184 

 Buckland (do.) 



s Thompson 



Dam of ice— >, 

 glacial dam.. 



Ja 



Jamieson 



Darwin 



LyeU 



Archibald Geikie 



j;ame5 Geikie 

 Sir Henry Ja 



vol. 

 1868 Geol. Soc. Lond. Jour., 



May, 1868. 

 i368 Geol. Soc. Lond. Jcur., 



August, 1868. 

 1S76 Proc. Roy. Soc. Edin., 



vol. viii., March, 1S76. 

 1879 Geol. Mag., Dec, 1879. 

 1823 Trans. Roy. Soc. Edin., 



vol. ix. 



1847 Proc.Roy.Soc.Edin., 1847. 



1876 Trans. Roy. Soc, vol. 

 .t.wii., 1876. 



1877 Trans. Roy. Soc, vol, 

 xxviii., part I, 1877. 



fGeol. Soc. Lond. Jour., 

 1842] vol. iii. 1842 : "Atlantic 

 1S42I Monthly," June, 1864 



I (Both by Agassiz). 



1848 Edin. New Phil. Jour., 



1863 Geol. Soc. Lond., vol. xi.\., 

 January, 1863. 



1863 In private letters. &c. 



1863 " Antiquity of Man." 



1865 " Scenery and Geology of 

 Scotland." 



1873 Trans. Geol. Soc. Edin., 

 April, 1S73. 



1873 *' The Great Ice Age," 



1S74 " Parallel Roads of Loch- 

 aber," Ord. Sur. Off. 



1876 Roy. Inst., June, 1876; 

 " Pop. Science Review,'' 

 October, 1876. 



1879 Roy. Soc.Lond.; Nature, 

 May2g.iS79, in abstract; 

 Lond. Phil. Trans.,i8So, 

 in full. 



III. — The Solutions Examined 

 After referring to the Traditional theories of Fingal and the 

 Hunting Roads, adopted by Pennant in his remarkable "Tour," 

 published in 1771, he told how Playfair had seen similar appear- 

 ances exhibited by irrigation works at Brieg, in the Valais, 

 which suggested to him his curious solution. 

 -■The Diluvial theory held that they were caused by an immense 

 flood from the Atlantic, through a sinking of the West Coast, 

 rushing impetuously along these valleys. There was no use 

 seriously criticising this theory, though adopted in 1S61 by Prof. 

 Rogers, of Glasgow. 



The Marine theory had had many able supporters, from its 

 first suggestion by Darwin, in 1839, to CampbeU of Islay, so 

 recently as 1877. The greatest exponent of this theory was 

 Robert Cliambers, in his "Ancient Sea Margins," published in 

 184S. He held that these Unes were nothing but sea beaches, 

 similar to those found so plentifully all over the country. He 

 contended that other lines in these same glens were of the same 

 kind ; but these had been shown by Mr. Jolly, in 1S73, to be 

 entirely different in character, outline, and composition, and 

 were probably moraines. Mr. Jolly then traversed the Conditions 

 of the problem laid down on the diagram above, and showed how 

 this theory violated, or failed satisfactorily to account for, Nos. 

 I. 4. 5. 7. 9, II, 12, 13, 14, 15. 16, 17, iS, 19, and 21, each of 

 v hich may be tested by the reader. 



■The Lacustrine theory would be found not open to the same 

 objections. The great difficulty here was the nature of the dam, 

 or barrier, that confined the waters of the lakes, of wliich the 

 famous Roads were the successive shores or beaches. This theory 

 was propounded, in 1S17, by the far-seeing Maccullocb, the eminent 



geologi^t, and early delineator of Highland scenery and geology ; 

 but he, along with others, had not condescended on the kind of 

 barrier required. Two styles of d.am had been contended for, 

 the one of detritus, the other of ice. The Detrital dam, first 

 suggested by Dick-Lauder in 1S23, had been adopted by Milne- 

 Home, who accompanied Robert Chambers to the region in 1S47, 

 and had written of it then, and twice since, with an amount cf 

 observation and detail tliat were of great and permanent value. 

 These writers held that tlie lakes were contained by huge banks 

 of debris, deposited by the sea and other causes, similar to that 

 existing abundantly in many parts of the country. There was 

 no doubt whatever that a large number of lakes, past and present, 

 had been dammed back by such a barrier in many places ; but if 

 the roads were so formed, why were not such remarkable lines 

 (whose character was unique) found elsewhere? Mr. Jolly 

 here examined this theory in detail, in connection with the 

 requisite Conditions, and held that it failed to satisfy many of 

 them. Howw-ere the-:e barriers so conveniently deposited at the 

 required points, \\hen the Great Glen, and the other valleys 

 were, according to Milue-IIome, filled with similar detritus ? In 

 the other lakes adduced by him, the outlets were oz'er the dJbiis, 

 gradually wearing it away, while here they existed at the upper 

 ends, flowing over hard rock. The roads ended abruptly on 

 the hill face, with no remains of the asserted barriers, heaped up 

 at ihdr extreviitUs, as might be expected, and was almost 

 universal. How were the requisite great accumulations so 

 effectively removed, reaching, as these must have done, to above 

 1,300 feet? ,The successive roads were on the same hill-face ; so 

 that tlie damming d^-bris must have been wholly removed 

 between the lines at their lower ends, before the new beaches 

 were laid down. Mr. Jolly entered into other difficulties attending 

 this theory, and finally concluded against it. 



TV. — The Glacial Theory, as adopted 

 The Glacial theory was started by Agassiz, the great Swiss, 

 who had been accustomed to the work of glaciers, after a visit 

 paid to Lochaber in company with Buckland, in 1840. It has 

 had the greatest number of adherents, Darwin and Lyell having 

 also given up the Marine theory for it. Its chief exponent was 

 Jamieson of Ellon, in an admirable paper published in 1863. 

 Mr. Jolly, by means of a large survey map, variously coloured, 

 entered into a careful explanation of the glaciation of the region. 

 He held that the roads were produced by lakes dammed back by 

 glacial ice, filling the lower parts of the valleys up to the ends of 

 the roads, and gradually retreating with the ameliorating climate, 

 at the last stage of the second portion of the Glacial epoch, imme- 

 diately before the final disappearance of local glaciers from Scot- 

 land. The abundant rolled dBris at the bottom of these valleys 

 was laid down by former glaciers, and by the sea during the great 

 depression in the middle of the Glacial period. He described the 

 peculiar configuration of the Ben Nevis Range, with its parallel 

 system of valleys opening out, on the south, to the close, deep 

 Glen Nevis and its eastern continuation, and, on the north, to the 

 broad Glen Spean and broader Glen More. The peculiarity of Glen 

 Spean was that it would receive not only the abundant ice from 

 the glens opening directly on it on the south, but also the greater 

 part of the ice accumulated in Glen Nevis and its continuation, 

 by the two outlets of Loch Treig and Glen Nevis itself. By 

 this means, and by its special relation to the highest mountains in 

 Scotland, it would receive an unusual supply of ice, equalled by 

 no other valley in the country. This was proved not only by its 

 geogi-aphical conformation, but by the superabundant glacial 

 remains in the district, of which Mr. Jolly gave full details. 

 During the first period of greater glaciation, the ice from Loch 

 Treig, after entering Glen Spean, turned east down Loch Lag- 

 gan, and west down the Spean ; while Glen Roy itself \\'as filled 

 with an ice-stream from the same valley, which moved out at its 

 head, down the Spey, as shown convincingly by the ice-markings 

 there ; and the Great Glen and its side valleys were also swathed 

 in ice. As the climate improved at the close of the ice period, the 

 glaciers gradually shrank backwards to their sources in the high 

 Nevis glens, which, from their altitude and neighbourhood to 

 the vapour-feeding Atlantic, would be the last in the country to 

 preserve local glaciers. At that time, from its peculiar relations 

 to these glens. Glen Spean would be filled from its head to the 

 sea with a great ice-stream, resting on the dibris already de- 

 posited by the sea, &c., and moving slowly downwards. This 

 stream, entering the south end of Glen Roy, dammed back a 

 lake there, fed by its tributai-y torrents, which has left its traces 

 in the roads. As the ice gradu.aUy shrank in successive steps, 

 the water subsided and the lake extended, as shown by these 



