94 



KNOWLEDGE. 



[Apeil 1, 1897. 



" outliers." Thug the Marlborough Downs, the Chiltern 

 Hills, the North and the South Downs, and the chalk 

 hills forming the backbone of the Isle of Wight, are all 

 of post-Eocene age. The last-named, at Whitecliff Bay 

 and at the Needles, may be seen to be almost vertical, 

 with the whole of the Lower Eocenes sharing their high 

 angle of inclination. 



WALES. 



The Snowdonian mountains (Snowdon, three thousand 

 five bundled and seventy-one feet) have at their loftiest 

 summit rocks of Bala or Lower Silurian age, and they are 

 there the remains of a vast synclinal fold the greater 

 portion of which has been removed by denudation ; so that i 

 possibly no less than ten thousand feet thickness of rocks 

 have been destroyed that were above what is now the summit 

 of the king of the Cambrian mountains. Associated and 

 interbedded with these sedimentary rocks are great sheets — 

 or, rather, masses — of Igneous rocks, which, together with 

 the Bala rocks, have been upheaved to great heights above 

 their original level, and so are unlike the volcanic 

 Cumbrian masses, which have been accumulated at an 

 original high elevation. The lower beds of the Llandovery 

 rocks, abutting on and resting unconformably on the 

 Snowdonian rocks, are conglomerates, and so show shallow 

 water conditions, from which it may be concluded that the 

 elevation of the mass was towards the close of the Lower 

 Silurian period. Many changes of level have doubtless 

 'occurred since; and there is evidence in the shells of recent 

 'age on Moel Tryfian, at an elevation of one thousand three 

 hundred feet, that the whole of Snowdonia below that 

 level has been below the sea-level in Pleistocene or 

 Quaternary times. 



south-west conformably by the Carboniferous rocks. The 

 age of the elevation must therefore be post-Carboniferous. 



SCOTLAND. 



In Scotland the north-western mountains (Scour Ourau, 

 three thousand and five feet) are separated from the more 

 central Grampians by Gen More, or the Great Glen, 

 extending across Scotland in a south-east to north-west 

 direction, from Loch Linnhe to the Moray Firth, at 

 Inverness. These mountains possess very great geological 

 interest, from displaying the oldest rocks in the British 

 Islands. They consist of crystalline schists, gneiss, quartz- 

 ites, and various granites, all highly metamorphosed rocks 

 that have been elevated, folded, contorted, and penetrated 

 by Igneous rocks before the overlymg strata were deposited. 

 These are red and purple sandstones (Torridon Sandstone) 

 of Cambrian age, which rise to considerable elevations, and 

 dip to the east. They are overlaid in turn, and uncon- 

 formably, by a series of Calcareous rocks, some dolomitic. 

 These rocks are known to be of Lower Silurian age, and 

 to the east pass under the Old Bed Sandstone. Con- 

 glomerates in the Lower Old Bed contain not only pebbles 

 of the older rocks, but fragments of considerable size, and 

 even large masses. The elevation of these mountains 

 must therefore be determined to be consequent upon 

 several movements at widely separated periods : firstly 

 pre-Cambrian, then pre-Bala, and then pre-Devonian. A 

 very remarkable feature of these mountains is a most 

 extensive overthrust, whereby the older rocks have been 

 made to overlie the Silurians. This renders the geology 

 here very complicated and difficult. 



The central mountains of Scotland, called the Grampians, 

 lie between the Great Glen and the valley extending across 



''ufamit S.ilt feet 



Section tlirough Snowdou. 1. Llaudeilo. 2. Cara^^oc or Bala. 3. Felspatliic Trap. 4. Volcanic Conglomerate. 



5. Intru -ve Igneous Kocks. 



The mountains of Central Wales are apparently so 

 confused that they, to an ordinary observer, seem to be 

 incapable of arrangement even physiographically. They 

 have, however, been interpreted chiefly through the early 

 work of Sedgwick and Murchison. They consist of 

 Silurian rocks flanked on their north-western side by 

 Carboniferous rocks, and the Silurians in Merionethshire 

 form a great anticlinal which is the complement of the 

 Snowdonian synclinal. Associated with the Silurian rocks, 

 especially with the Llandeilo, are vast interbedded masses 

 of volcanic rocks and traps, which, forming very steep 

 ascents and perpendicular precipices, give a rugged and 

 wild aspect to the scenery comparable with the volcanic 

 area of Cumberland. As the Carboniferous Limestone on 

 the west in Denbighshire and Flintshire rises to a very 

 considerable elevation, it must have shared in the general 

 elevation, and so the central mountains of Wales may be 

 considered to be post-Carboniferous and pre-Triassic. 



The southern mountains of Wales include the Black 

 Mountains with the Fans of Brecon (summit elevation, 

 two thousand eight hundred and sixty-two feet). They 

 are formed of the Old Red Sandstone, which here has a 

 thickness of fully ten thousand feet, and is covered on the 



the island from the Firth of Forth to the Firth of Clyde. 

 Although these are formed in the main of highly altered 

 Lower Silurian rocks also, they have several features in 

 which they difl'er from the more northern mountains. The 

 intrusive Igneous rocks are much more important, forming 

 as they do vast masses that tower to great elevations, 

 and form important mountain summits ; one of these is 

 the highest land in the British Islands, Ben Nevis 

 (four thousand four hundred and seven feet). Denudation 

 has sculptured the whole mass into more varied contours, 

 which give more rugged scenery. This sculpturing has, 

 however, been commenced on a tableland, as is apparent 

 when an extensive distant view is obtained of these 

 mountains, for then it is seen that the summits rise to a 

 straight line with remarkable uniformity. Again, the 

 watershed is not so near the sea on the west, though 

 still much nearer than to the east coast. Old Red 

 Sandstone, with basal beds of coarse conglomerate, lies 

 unconformably on the Silurians to the east, from which 

 we may place the period of the great uplift before the 

 deposition of those rocks. 



The southern mountains of Scotland (Mount Merrick, two 

 thousand seven hundred and sixty- four feet) are likewise 



