PRESIDENTIAL ADDRESS. 411 



none the less appear catastrophic in their intensity. Were they subterranean, 

 they would be felt at the surface in more or less degree, according to the 

 depth of the melting zone. In early pre-Cambrian times, whether a cooling 

 molten earth was resenting its first imprisonment in a crust, or whether the 

 collision of meteorites or the concentration of radio-active matter towards the 

 surface was beginning to make itself effective, the zone of melting appears to 

 have influenced great thicknesses of overlying strata. This influence was not 

 world-wide at one and the same epoch, as we may see by the preservation of 

 .slightly altered sediments in certain places ; but it was vigorous, menacing, 

 and recurrent. 



Under such conditions, even the surface-rocks must have fallen in at some 

 points and have been replaced by igneous extrusions. Iscstatic adjustments 

 must have been very frequently disturbed. Folding of rocks, as a phenomenon 

 of lateral surge and flow, must have made itself freely felt at the earth's surface. 

 It is safe to assert that such conditions have not been repeated on a broad scale 

 at any geological period subsequent to the spread of the Olenellus-fauna. Geo- 

 chemical evolution, however, may have surprises still in store, and, in spite of 

 long tradition, we are disinclined nowadays to rely too strongly on arguments 

 based upon the sanctity of human life. 



Possible Breaks in the Slow Continuity of Earth-moveincnt. 

 1. The Mountain-building Stage. 



Even with the thickened sedimentary crust beneath us, and the confidence 

 inspired by our limited experience of the earth, we may ask if subterranean 

 changes may not still result in catastrophes at the surface. Volcanic paroxysms 

 have been regarded complacently as safety-valves, and the destruction of thirty 

 thousand persons in a few minutes in Martinique or the Straits of Sunda form 

 interesting historic episodes, when viewed from the platform or the pulpit of 

 survivors in other lands. The ' grand theatre of action,' as Lyell says, may 

 sliif t ; but we feel that it will not do so in our own time. Some people live 

 under towers of Siloam, others in San Francisco ; but, after all, the menace 

 appears small to the teeming hmnanity of the earth. The rise of an imperial 

 dynasty at our side in Europe is more to be dreaded than that of isogeotherms 

 beneath us. 



What, however, is likely to occur if a mountain-building episode again sets 

 in? Such episodes, affecting very wide areas, have undoubtedly recurred in the 

 earth's history. We do not know if they are rhythmic ; we do not know if they 

 represent a pulsation, decreasing in intensity, inherited from the stars and 

 hampered by increasing friction ; we do not know if they record internal 

 chemical changes, which have no climax, because they are neither cyclic nor 

 involutionary, but evolutionary. The mid-Huronian chains, now worn down and 

 supplying such valuable horizontal sections, were evidently of great extent ; but 

 we cannot say that they were vaster than those of later limes. 



The phenomena accompanying the growth of a single chain in the Cainozoic 

 era give us, at any rate, ample food for thought. Though tbe narrow cross- 

 section of the core of such a chain limits our field of observation, the same 

 inpressings of igneous material, and the same features of rock-weakening and 

 rock-destruction, may be observed as in the immense basal sections exposed in 

 the Archaean platforms. The progress of geological time has not diminished the 

 activity in the depths. The granodiorite of western Montana,^' for instance, 

 which intruded during an uplift in early Eocene times, has attacked the 

 Algonkian sediments of the district, producing phenomena of stoping and 

 assimilation in the true ' Laurentian ' style. 



In the western and central Alps, again, the absence of any fossiliferous strata 

 older than the Carboniferous arouses some surprise, until we find that many 

 of the granitic intrusions are of late Carboniferous age. The crystalline schists 



'' J. Barrell, ' Marysville district, Montana ; a study of igneous intrusion and 

 contact metamorphism,' U.S. Geol. Surv., Prof. Paper 57 (1907), and W. H. 

 Emmons and F. C. Calkins, ' Phillipsburg Quadrangle,' ihicL, Paper 78 (1913). 



