738 



SCIENCE 



[N. S. Vol. XXVIII. No. 726 



certainty that the tireless forces of denuda- 

 tion labored to undo their former work. 

 Each era represents a slow and measured 

 pulse in the earth's crust, as if the over- 

 loading and sinking of the surface materials 

 induced the very conditions required for 

 their reelevation. Such events, even in 

 times when the crust was thinner and more 

 readily disturbed than it is now, must have 

 taken vast periods of time. The uncon- 

 formity may represent as long a period as 

 that of accumulation. In these Protero- 

 zoic areas of America, as elsewhere on the 

 globe and throughout the whole of geolog- 

 ical history, there has been a succession in 

 time of foldings of the crust always so 

 located as to uplift the areas of sedimenta- 

 tion, these upheavals being sundered by 

 long intervals during which the site of sedi- 

 mentation was transferred and preparation 

 made for another era of disturbance. 

 However long deferred, there seems to be 

 only the one and inevitable ending, in- 

 ducing a rhythmic and monotonous repeti- 

 tion surely indicative of some cause of in- 

 stability 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 pre- 

 ceding era; beds that were laid down conformably, 

 and in succession, until they had reached the 

 needed thickness; beds spreading over a region 

 tens of thousands of square miles in area. The 

 region over which sedimentary formations were 

 in progress in order to make, finally, the Appa- 

 lachian range, reached from New York to Ala- 

 bama, and had a breadth of 100 to 200 miles, and 

 the pile of horizontal beds along the middle was 

 40,000 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 in a 

 trough 40,000 feet deep, filling the trough to the 

 brim. It thus appears that epochs of mountain- 



making have occurred only after long intervals 

 of quiet in the history of a continent. 



The generally observed fact that the 

 deposition of sediments in some manner 

 involves their ultimate upheaval has at 

 various times led to explanations being 

 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 satis- 

 factory explanation of the connection be- 

 tween sedimentation and upheaval has been 

 advanced. The mere shifting upwards of 

 the isogeotherms into the deposits, ad- 

 vanced as a source of local loss of rigidity 

 by Babbage and Herschel, need not involve 

 any such loss so long as the original dis- 

 tance of the isogeotherms from the surface 

 is preserved. 



We see in every case that only after 

 great thickness of sediments has accumu- 

 lated is the upheaval brought about. This 

 is a feature which must enter as an essen- 

 tial 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 endeavor to form some 

 approximate estimate of the rise of tem- 

 perature which will be brought about at 

 the base of such great sedimentary accu- 

 mulations 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 posi- 

 tion relative to the normal isogeotherms, 

 whether these latter are in turn due to or 

 influenced by radio-thermal supplies or not. 

 It is convenient, and I think allowable, to 

 consider these two effects separately, and 

 deal with them as if they were independent, 



