224 



NATURE 



[July 7, 1892 



ON THE CA USES OF THE DEFORM A TION OF 

 THE EARTH'S CRUST. 



Mountain-making. 



BY eminent geologists it has been shown that the con- 

 traction hypothesis is not sufficient to account for 

 the observed deformations of the earth's crust. We are 

 obliged to look for other causes of deformation. 



The form of a cosmic body must be irregular if the 

 masses are unequally mixed. Already in the liquid stage 

 under this condition a geoid is formed. The radius with 

 dense material must be shorter, so much as to equilibrate 

 the higher regions with less density. 



This cause of constant irregularity is not sufficient to 

 explain the existing differences of level. In fact, depres- 

 sions and elevations are not the result of a constant equi- 

 librium ; they are not permanent. Sedimentation and 

 erosion disturb the mechanical and the thermal equi- 

 librium and cause a continual deformation of our planet. 

 Another cause of deformation is found in the continual 

 shifting of material. Accumulation of eruptive material 

 and of sediments (loading) on one side, and erosion (dis- 

 burdening) on the other side, cause deformations of the 

 earth's crust. If the plasticity of the cosmic body is 

 great, the surface of the burdened and disburdened 

 regions has the tendency to remain nearly level — a 

 quasi-hydrostatic (a " magmastatic ") equilibrium will 

 dominate. 



As the material of our earth is not very plastic, and as 

 other causes of deformation have a contrary effect, it is 

 natural that geological facts are not in accordance with 

 this hypothesis. 



Contradictory to this hypothesis are the facts (i) that 

 subsidence does not continue as long as sedimentation 

 goes on ; (2) that sinking often is considerable, though 

 the loading is slight ; (3) that in many cases enormous 

 loading does not produce a depression of the earth's crust 

 (volcanic chains growing up on a highland). 



The Thermal Theory. 



The constant disturbance of thermal equilibrium is of 

 the highest importance. Sedimentation causes an as- 

 cending movement of the geo-isotherms : expansion 

 and general elevation. If the dilatation is concentrated, 

 there may result a fold-chain (Hall, Reade). The 

 hypothesis is supported by the fact that the elevation 

 and folding always drives up sediments, which were 

 formed immediately before the orogonic movement. 

 The mountains grow up from a shallow sea, they are 

 never generated in the middle of a continent, which 

 might as well occur according to the contraction- 

 hypothesis. 



Messrs. Fisher, Hutton, and Reade have considered the 

 thermal effect, and agree that it is sufficient to produce 

 considerable deformations. But to produce a mountain- 

 chain of some 1000 m., we must suppose a concentration 

 of the effect in one zone, as long as we, according to Mr. 

 Reade, consider only the effect of thermal expansion in 

 the earth's crust. 



As physical geology considers the earth as a rigid body 

 (the plasticity, according to Mr. G. Darwin, being that of 

 steel) there is no reason why the thermal expansion ought 

 not to proceed through the rigid magma to the region 

 of constant temperature. The increase of temperature 

 being 3° C. for 100 m., the temperature at the depth of 

 40 km. = 1200° C, at 50 km. = 1500^ C. After sedimen- 

 tation of 10 km. the base of the sediments is warmer by 

 300°. The underlying masses are equally warmer by 

 this quantity. 



The linear expansion of rocks per 100'' C. is nearly = 

 I per mille, i.e. i metre per km. In our case the ex- 

 pansion is = 3 m. per km. Lateral expansion being 

 impossible, it results in a vertical elevation of nearly i per 



NO. I 184, VOL. 46] 



cent. The crust would be elevated through the full 

 expansion by 500 metres. 



If we consider the thermal expansion proceeding to a 

 depth of 500 or 1000 km. through the rigid magma, we 

 find that indeed highlands and chains of some 1000 m. 

 may be driven up, even if we do not suppose a concen- 

 tration of the thermal effect on a restricted zone. 



Yet certain facts are not in accordance with the theory 

 thus formulated. (i) Elevation and mountain-making 

 is not a slow and constant process, but it is executed in a 

 short time (relatively). (2) Folding in some cases does 

 not reach to a considerable depth, but we often meet 

 undisturbed masses below the folded complex. These 

 facts induce us to modify the hypothesis. 



Messrs. Gilbert and Suess have shown that the move- 

 ment of folding is horizontal and superficial ; we may 

 consequently ask whether folding may not be caused by 

 2, gliding mov^mtxvl (see my "Theoretical Geology"). 



If we deposit under water sediments of great plasticity, 

 and if we incline afterwards the masses to the extent of 

 5° or 10°, there succeeds a gliding movement, especially 

 if the sediments partly emerge from the water-level^ 

 and if occasional shaking (earthquakes) occurs. 



The gliding masses form a fold-chain. The Silurian, 

 of Christiania is intensely folded, but it rests on an un- 

 disturbed base (Brogger). The folded Jurassic strata of 

 the Weser chain likewise repose on an unfolded base. 

 In such cases it is impossible to derive folding from a 

 general contraction, nor can we explain the quiet base 

 by supposing a concentration of thermal expansion in 

 certain districts. The existence of a quiet base is ex- 

 plained only if we admit folding to be in such cases a 

 gliding process. 



The fact that folding in nature is accompanied by 

 emersion is in accordance with these views. 



Contradictory to this hypothesis seems the fact that the 

 hypothetical land (from which the folded sediments were 

 pushed towards the lowland) in the back of the chain 

 is often wanting, and that in its place a (marine or a 

 terrestrial) depression exists. This objection disappears 

 if we pursue the process, and we find that this seemingly 

 contradictory face indeed must result : partial cooling 

 causes local depression. Erosion has the same effect. 

 If I km. (vertical measure) of rock mass is denuded, the 

 temperature of the new surface is lower by 30" C. than it 

 was at this point before erosion occurred. This cooling 

 propagates into depth, and the denuded landgets depressed. 



The highland, from which the sediments glide away^ 

 must sink down in course of time. The Jura is pushed 

 towards the French plain ; in the back is situated the de- 

 pression of Neuchatel. Here, according to the deduc- 

 tion, existed a highland, which subsided in consequence 

 of cooling. Between the fold-chain and the depressed 

 district are situated deep ruptures, along which earth- 

 quakes occur as long as the depression goes on. 



East of the Appalachian Mountains, as late as the 

 end of the Palaeozoic era, a highland was situated, 

 wherefrom the detritus-masses were transported into the 

 Appalachian sea. Afterwards the Carboniferous emersion 

 occurred (in consequence of thermal expansion) and the 

 Palaeozoic sediments were pushed towards the western 

 lowland ; here the Appalachian chain was generated. 

 Erosion and consequent cooling, instead of the old eleva- 

 tion, caused a depression in the eastern region, which 

 got inundated by the ocean. 



In course of time the adjoining districts have changed 

 parts. In the lowland a chain is driven up and the old 

 highland sinks down. 



Eruptive districts form depressions with growing 

 accumulations. The thermal effect in course of time 

 leads to an opposite movement. Material of 1000° C. 

 flows through many fissures and covers the surface. The 

 eruptive region, in consequence, gets heated in a higher 

 degree than by simple sedimentation. The period of 



