NATURE 



265 



THURSDAY, JANUARY 19, li 



THE ANATOMY OF THE EARTH'S CRUST. 

 Earth Sculpture ; or, Ihe Orii^in of Land Forms. By 



James Geikie, LL.D., D.C.L., F.R.S. Pp. xvi + 3:0. 



(London : John Murray, 1898.) 



THE sculptor must be careful to mould his drapery so 

 that its fall and fold may accord with the form 

 below ; or, if he copies the undraped human form divine, 

 however quick his eye to detect and his hand to represent, 

 if he wishes to follow the curves of beauty with ease and 

 accuracy he must know the position and functions of the 

 muscles and bones upon which the outlines and pose of 

 the body depend. 



So in the study of scenery we shall be better able to 

 appreciate the forms presented to us if we understand the 

 causes which have determined them ; then we shall look 

 with ditterent eyes on the gently rounded outlines where 

 soft surface-drifts like drapery have clothed the solid 

 rocks, tilled up their hollows, and concealed their rugged- 

 ness. Still more shall we have the pleasures of imagin- 

 ation heightened when we can, from an examination of 

 the surface, realise the " ribs of porphyry " or the "joints 

 of the limestone " that have determined the configuration 

 and lie of the land before us. 



.\ very useful work for the sculptor would be " Outlines 

 of Human Anatomy for .Artists, with Dissections " ; but 

 that would not be a work on sculpture ; and the book 

 before us might not inappropriately have been called 

 " The Morphology and Physiology of the Earth's Crusty 

 with Sections ; being an Introduction to the Study of 

 Earth Sculpture and its resultant Scenic Features." For 

 it includes far more than its title would imply. It is, in 

 fact, a series of essays upon the principles of geology, in 

 which the author keeps in view the question which on his 

 title he has proposed for consideration ; and, at the same 

 time, gives great prominence to those aspects of the 

 subject which bear upon certain theories towards the 

 development and promulgation of which he has taken a 

 leading and distinguished part. 



He points out that subterranean action merely provides 

 the rough block which the surface agents of denudation 

 subsequently sculpture into shape, and that, with few 

 exceptions, the land features that now meet our eye are 

 the direct result of erosion and accumulation, the modify- 

 ing influence of which is always more or less conspicuous. 



As the work is intended for readers not skilled in 

 geology, the author has not thought it necessary to burden 

 the pages with references, which for the student are in- 

 dispensable. He has adopted the a /wr/ method ; and 

 a great part of the work is devoted to explaining what 

 might, could, would, or should be, assuming the prevalence 

 of certain conditions. 



The readjustments of a hardening crust to a shrinking 

 nucleus are referred to as sufficient cause of the foldings 

 which have lifted large areas within reach of earth 

 sculpture, and produced the crumplings and great variety 

 of structure observed in many mountain chains. Perhaps 

 mention might have been made of other theories to 

 account for crustal movements, such as the loading of 

 off-shore areas by sediment, and the corresponding 

 NO. 1525, VOL. 59] 



lightening of the adjoining areas from which that sedi- 

 ment was derived ; the theories of subterranean lakes of 

 molten matter ; the changes of volume which accompany 

 chemical, mineralogical, thermal and other changes in 

 the rocks, and so on. 



When looking at the subject from the point of view of 

 earth sculpture, the most important point is the more or 

 less yielding character of the rock — whether this be due 

 to its chemical composition as shown in the manner in 

 which a limestone is dissolved, and rocks with potash 

 felspars crumble away ; or whether we regard its texture 

 and structure as shown in the manner in which a shale 

 often resists denudation, while a tough massive rock 

 breaks along joints and bedding planes, and is thus 

 readily cut back ; or in its relation to the lie of the rocks 

 as shown in the way in which they resist denudation 

 better where they present solid bed-faces to the weather 

 than where the denuding agents can attack them along 

 the lines of weakness between the divisional planes. 

 Among beds which are horizontal, or only slightly and 

 uniformly inclined over large areas, there will be a 

 greater similarity in the resultant features than there can 

 be if the beds are thrown into sharp folds, so that rapid 

 alternations of rocks of different solubility, hardness, 

 &c., are exposed in ever-varying positions within short 

 distances. These points chiefly are elaborated in the 

 first nine chapters. 



Then we have two chapters on the modification of land 

 forms by glacial agency. The work of ice at the present 

 time is described and the traces of similar work in the 

 past, over areas from which the ice has long been re- 

 moved, are sketched out. It does not, however, neces- 

 sarily follow that it was more generally extended over 

 either hemisphere. We shall sufficiently account for all 

 the phenomena observed if we admit that the scene of 

 its severest operations has been shifted from time to 

 time. 



Among the controverted questions relating to the 

 origin of glacial accumulations is that of the mode of 

 formation of the ground moraine, that is the great mass 

 of clay and rock which is found at the base of the ice, 

 and of which relics are left plastered over the surface of 

 many glaciated regions. 



Have the larger fragments, at any rate, worked their 

 way down from moraines and from the sides to the 

 bottom of the ice, where, crushed against one another 

 and driven over the underlying rock, they produce the 

 clay which forms the matrix of the ground moraine and 

 the "flour of rock" which discolours glacial streams? 

 Or is the ground morame derived chiefly from the rock 

 over which the ice is travelling ; does little material 

 reach the bottom of the glacier or ice-sheet from its 

 surface ; and has the ice the power of extracting pieces 

 of rock from its bed, and using them as tools to plough 

 up or grind away more .' 



This last is the view which our author favours, but it 

 involves the concession that some rock fragments mus 

 have got in from above to start the work, as ice, like 

 water in this respect, has practically little eroding power, 

 but moulds itself round obstructions and only operates in 

 denudation as a handle or back to hold the fragments of 

 rock which form the rasp that really does the work. 



Many interesting examples are cited of pieces of rock 



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