376 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1908. 



The history of mountain building has repeated itself many times; 

 ages of sedimentation, with attendant sinking of the crust in the area 

 of deposition, then upheaval, folding up of the great beds of sedi- 

 ment, and even their overthrusting for many miles. So that the 

 mountain ranges of the world are not constituted from materials 

 rising from below, save in so far as these may form a sustaining 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 underestimate 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 

 Huronian, Animikean, and Keweenawan ages, where deposits meas- 

 ured in miles of thickness are succeeded by unrecorded intervals of 

 time, in which we know with certainty that the tireless forces of 

 denudation labored 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 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 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 preparation made for an- 

 other era of disturbance. However long deferred, there seems to be 

 only the one and inevitable ending, inducing a rhythmic and monoto- 

 nous repetition surely indicative of some cause of instability attend- 

 ing the events of deposition. 



The facts have been impressivelj'^ stated by Dana : 



A mountain range of the common type, like tli,at to which the Appalachiiuis 

 belong, is made out of the sedimentary formations of a long-preceding 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 Appalachian 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 .Moun- 

 tains 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 



