458 Notices of Memoirs — Australia, 1914- — ■ 



respectable life of any scientific theory. " For the condition of equilibrium of 

 figure, to which gravitation tends to reduce a planetary body, irrespective of 

 whether it be homogeneous or not," Button^ proposed " the name isostasy " . 

 The corresponding adjective would be isostatic — the state of balance between 

 the ups and downs on the Earth. 



For a long time geologists were forced to content themselves with the con- 

 clusion that the folding of strata is the result of the crust collapsing on a cooling 

 and shrinking core ; but Fisher pointed out that the amount of radial shrinking 

 could not account even for the present great surface inequalities of the litho- 

 sphere, without regard to the enormous lateral shortening indicated by the 

 folds in great mountain regions, some of which, like the Himalayan folds, were 

 formed at a late date in the Earth's history, folds which in date and direction 

 have no genetic relationship to G. H. Darwin's primitive wrinkles. Then, 

 besides the folding and plication of the crust in some areas, we have to account 

 for the undoubted stretching which it has suffered in other places, stretching 

 of a kind indicated by faults so common that they are generally known as 

 normal faults. It has been estimated by Claypole that the folding of the 

 Appalachian Range resulted in a horizontal compression of the strata to a belt 

 less than 65 per cent of the original breadth. According to Heim the diameter 

 of the northern zone of the Central Alps is not more than half the original 

 extension of the strata when they were laid down in horizontal sheets. 

 De la Beche, in his memoir on Devon and Cornwall, which anticipated many 

 problems of more than local interest, pointed out that, if the inclined and 

 folded strata were flattened out again, they would cover far more ground than 

 that to which they are now restricted on the geological map. Thus, according 

 to Dutton, Fisher, and others, the mere contraction of the cooling globe is 

 insufficient to account for our great rock-folds, especially great folds like those 

 of the Alps and the Himalayas, which have been produced in quite late 

 geological times. It is possible that this conclusion is in the main true ; but 

 in coming to this conclusion we must give due value to the number of patches 

 which have been let into the old crustal envelope — masses of igneous rock, 

 mineral veins, and hydrated products which have been formed in areas of 

 temporary stretching, and have remained as permanent additions to the crust, 

 increasing the size and bagginess of the old coat, which, since the discovery of 

 radium, is now regarded as much older than was formerly imagined by non- 

 geological members of the scientific world. 



The peculiar nature of rock-folds presents also an obstacle no less formidable 

 from a qualitative point of view. If the skin were merely collapsing on its 

 shrinking core we should expect wrinkles in all directions ; yet we find great 

 folded areas like the Himalayas stretching continuously for 1,400 miles, with 

 signs of a persistently directed overthrust from the north ; or we have folded 

 masses like the Appalachians of a similar order of magnitude stretching from 

 Maine to Georgia, with an unmistakable compression in a north-west to south- 

 east direction. The simple hypothesis of a collapsing crust is thus " quantita- 

 tively insufficient ", according to Dutton, though this is still doubtful, and it is 

 " qualitatively inapplicable ", which is highly ]Drobable. 



In addition to the facts that rock-folds are maintained over such great 

 distances and that later folds are sometimes found to be superimposed on older 

 ones, geologists have to account for the conditions which permit of the gradual 

 accumulation of enormous thicknesses of strata without corresponding rise of 

 the surface of deposition. 



On the other hand, too, in folded regions there are exposures of beds super- 

 imposed on one another with a total thickness of many miles more than the 

 height of any known mountain, and one is driven again to conclude that uplift 

 has proceeded pa7-i passu with the removal of the load through the erosive work 

 of atmospheric agents. 



It does not necessarily follow that these two processes are the direct result 

 of loading in one case and of relief in the other ; for slow subsidence gives rise 



^ Dutton, " On some of the Greater Problems of Physical Geology": Bull. 

 Phil. Soc. Washington, vol. xi, p. 53, 1889. 



