510 British Association Reports. 



Silurian rocks amid which: they lie ; and the mountainous character 

 of the country, as it now presents itself, is due, not to direct volcanic 

 action, but to the unequal hardness of igneous and aqueofis masses, 

 acted on by many denudations both ancient and modern, both marine 

 and subaerial. These causes, aided by faults which often brought 

 hard and soft rocks into immediate juxtaposition, have given rise to 

 all the rugged outlines on the surface of Wales, the hard rooks more 

 strongly resisting decay and waste, the soft ones yielding to time, 

 the sea, and the weather, with greater ease ; and thus it happens that 

 the harder masses generally form headlands, and the summits of the 

 mountains, though often found elsewhere ; while the softer strata, 

 wasted away by the sea and by rain and rivers, are apt to lie in the 

 recesses of bays and valleys. This kind of argument I could equally 

 well apply to the Carboniferous formations of Scotland, where 

 igneous rocks are rife, and, indeed, to all those areas where igneous 

 masses of ancient date are found intermixed with sedimentary 

 strata.^ 



Again, if we go to the Alps, and look at the strata there, which 

 are disturbed on the greatest scale ; in all that part of the range that 

 I best know, from east to west for more than 100 miles in length, I 

 have never seen a fragment of what I can call a true igneous rock. 

 Gneiss there is, and granite there is, which, according to old ideas, 

 a great advance in their day, some have been apt to classify either 

 as common igneous productions or as closely allied to them ; but no 

 basalts or greenstones, or rocks allied to them, play any important 

 part in the structure of the country, although the strata have been 

 disturbed in a manner of which no conception can be formed by 

 those who have only studied such minor mountains as those of the 

 British Isles. There, in the Alps, we find areas as large as half an 

 English county, in which a whole series of formations has been 

 turned upside down. But by what means were masses of strata 

 many thousands of feet thick bent and contorted and raised into the 

 air so as to produce such results and thus affording matter for the 

 elements to work upon ? Not by igneous or other pressure and 

 upheaval from below, for that would stretch instead of crumiiiling the 

 strata in the manner we find them in great mountain chains like the 

 Alps, or in less disturbed groups like those of the Highlands, 

 Wales, and Cumberland, which are only fragments of older mountain 

 ranges ; but, perhaps, as some have supposed, because of the ra- 

 diation from the earth of heat into space, producing gradually a 

 marked shrinkage of the earth's hardened crust, which, giving way, 

 became crumpled along lines more or less irregular, those producing 

 partial upheavals, though the bulk of the whole globe was diminishing. 

 A modification of this hypothesis does not attempt to explain the 

 positive cause of the shrinkage, but simply states, that from some 

 unknown cause, irrespective of radiation, great areas of the earth's 

 crust having been depressed, broad lines that lie between them have 

 been contorted and heaved into the air in the manner already indi- 



1 This argument has of course no immediate application to existing or late Tertiary- 

 volcanic areas, such as those of Auvergne, where entire and ruined craters still exist. 



