464 Notices of Meiinoirs — Address by Si7^ T. H. Holland. 



no strain ". Above this level there should be a shell of compression, and under 

 it a thicker shell of tension. The idea has been treated mathematically by 

 C. Davison, G. H. Darwin, O. Fisher, and M. P. Rudski, and need not be discussed 

 at present. Professor E. A. Daly has taken advantage of this view concerning 

 the distribution of stresses in the crust to explain the facility for the injection 

 of dykes and batholiths from the liquid, or potentially liquid, gabbroid magma 

 below into the shell of tension.^ He also shows that the injection of large 

 bodies of basic inaterial into the shell of tension tends on purely mechanical 

 grounds to the formation of a depression or geosyncline. If this be so, are we 

 justified in assuming that the heavy band following the southern margin of the 

 Gangetic geosyncline is a ' range ' of such batholiths ? The idea is not entirely 

 new ; for 0. Fisher made the suggestion more than twenty years ago that the 

 abnormal gravity at Kalianpur was due to ' ' some peculiar influence (perhaps 

 of a volcanic neck of basalt)".- 



Daly's suggestion, however, taken into account with th,e history of Gondwana- 

 land, may explain the peculiar alignment of the heavy subterranean band, 

 parallel to the Gangetic depression and parallel to the general trend of the 

 peninsular tension-faults and fissures that followed the unloading of Gondwana- 

 land and the heavy loading of the adjoining ocean bed along a band roughly 

 parallel to the present Himalayan folds. 



E. S. Woodward objected that isostasy does not seem to meet the requirements 

 of geological continuity, for it tends rapidly towards stable equilibrium, and the 

 crust ought therefore to reach a stage of repose early in geologic time.^ If the 

 process of denudation and rise, with adjoining deposition and subsidence, 

 occurred on a solid globe, this objection might hold good. But it seems to ms 

 that the break-up of Gondwanaland and the tectonic revolutions that followed 

 show how isostasy can defeat itself in the presence of a sub-crustal magma 

 actually molten or ready to liquefy on local relief of pressure. It is possible 

 that the protracted filing off of Gondwanaland brought nearer the surface what 

 was once the local level of no strain and its accompanying shell of tension. 



The conditions existing in Northern Gondwanaland before late Mesozoic times 

 must have been similar to those in South- West Scotland before the occurrence 

 of the Tertiary eruptions, for the crust in this region was also torn by stresses 

 in the S.W.-N.E. direction with the formation of a remarkable series of 

 N.W.-S.E. dykes which give the 1 in. geological maps in- this region 

 a regularly striped appearance. 



There is no section of the Earth's surface which one can point to as being 

 now subjected to exactly the same kind and magnitude of treatment as that to 

 which Gondwanaland was exposed for long ages before the outburst of the 

 Deccan Trap ; but possibly the erosion of the Brazilian highlands and the 

 deposition of the silt carried down by the Amazon, with its southern tributaries, 

 and by the more eastern Araguay and Tocantins, may result in similar stresses 

 which, if continued, will develop strains, and open the way for the subjacent 

 magma to approach the surface or even to become extravasated, adding another 

 to the small family of so-called fissure-eruptions. 



The value of a generalization can be tested best by its reliability as a basis 

 for prediction. Nothing shows up the shortcomings of our knowledge about the 

 state of affairs below the superficial crust so effectually as our inability to make 

 any useful predictions about earthquakes or volcanic eruptions. For many 

 years to come in this department of science the only worker who will ever 

 establish a claim to be called a prophet will be one in Cicero's sense — "he who 

 guesses well." 



^ E. A. Daly, "Abyssal Igneous Injection as a Causal Condition and as an 

 Effect of Mountain-building " : Amer. Journ. Sci., vol. xxii, p. 205, September, 

 1906. 



- Physics of the Earth's Cnist, 2nd ed., 1889, p. 216. 



^ Address to the Section of Mathematics and Astronomy of the Amer. 

 Assoc, 1889. Smithsonian Eeport, 1890, p. 196. 



