PRESIDENTIAL ADDRESS. 691 



Bustaininf^ core, but of the slowly accumulating deposits of the ages preceding the 

 upheaval. 



The thickness of collected sediments involved in these great events is enormous, 

 and although uncertainty often attends the estimation of the aggregate depths of 

 sedimentation, yet when we consider that unconformities between the deposits of 

 succeeding eras represent the removal of vast masses of sediment to fresh areas 

 of deposition, and often in such a way as to lead to an under-estimate of the 

 thickness of deposit, the observations of the geologist may well indicate the minor 

 and not the major limit. Witness the mighty layers of the Hurouian, Animikean, 

 and Keweenawan ages where deposits measured in miles of thickness are succeeded 

 by unrecorded intervals of time, in which we know with certainty that the tireless 

 forces of denudation laboured to undo their former work Each era represents a 

 slow and measured pulse in the earth's crust, as if the overloading and sinking of 

 the surface materials induced the very conditions required for their re-elevation. 

 Such events, even in times when the crust was thinner and more readily disturbed 

 than it is now, must have taken vast periods of lime. The unconformity may 

 represent as long a period as that of accumulation. In these Proterozoic areas 

 of America, as elsewhere on the globe and throughout the whole of geological 

 history, there has been a succession in time of foldings of the crust always so 

 located as to uplift the areas of sedimentation, these upheavals being sundered by 

 long intervals during which the site of sedimentation was transferred and pre- 

 paration made for another era of disturbance. However long deferred there seems 

 to be only the one and inevitable ending, inducing a rhythmic and monotonous 

 repetition surely indicative of some cause of instability attending the events of 

 deposition. 



The facts have been impressively stated by Dana : ' A mountain range of the 

 common type, like that to which the Appalachians belong, is made out of the 

 sedimentary formations of a long preceding era ; beds that were laid down con- 

 formably, and in succession, until tliey had reached the needed thickness : beds 

 spreading over a region tens of thousands of square miles in area. The re£rion 

 over which sedimentary formations were in progress in order to malce, finally, the 

 Appalachian range, reached from New York to Alabama, and had a breadth 

 of 100 to 200 miles, .and the pile of horizontal beds along the middle was 10,000 feet 

 in depth. The pile for the Wahsatch Mountains was 60,000 feet thick, according 

 to King. The beds for the Appalachians were not laid down in a deep ocean, but 

 in shallow waters, where a gradual subsidence was in progress ; and they at last, 

 when ready for the genesis, lay iu a trough 40,000 feet deep, filling the trough 

 to the brim. It thus appears that epochs of mountain making have occurred onh 

 after long intervals of quiet in the history of a continent.' 



The generally observe! fact that the deposition of sediments in some manner 

 involves their ultimate upheaval has at various times led to explanations bein" 

 offered. I think I am safe in saying that although the primary factor, the com- 

 pressive stress in a crust which has ceased to fit the shrinking world within it, 

 has probably been correctly inferred, no satisfactory explanation of the connec- 

 tion between sedimentation and upheaval has been advanced. The mere shifting 

 upwards of the isogeotherms into the deposits, advanced as a source of local loss 

 of rigidity by Babbage and Herschel, need not involve any such loss so long as the 

 original distance of the isogeotherms from the surface is preserved. 



We see in every case that only after great thicknesses of sediments have accu- 

 mulated is the upheaval brought about. This is a feature which must enter as 

 an essential condition into whatever explanation we propose to offer. 



Following up the idea that the sought-for instability is referable to radio- 

 thermal actions, we will now endeavour to form some approximate estimate of 

 the rise of temperature which will be brought about at the base of such great sedi- 

 mentary accumulations as have gone towards mountain building, due to the radium 

 distributed throughout the materials. 



The temperature at the base of a feebly radio-active layer, such as an accumu- 

 lation of sediments, is defined in part by radio-active energy, in part by its position 

 relative to the uorroal isogeotherms, whether these latter are in turn due to or 



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