204 N. S. SHALER FORMATION OF CONTINENTS. 



the topographic relief which would be given* them by restoring the strata 

 which once overarched their summits, we may still believe that if they 

 developed on the ocean-bottom as they do on the great lands they would 

 attain on the average their greatest elevation in the sea. We may there- 

 fore assume that there is some causal relation between the growth of 

 continents and the dislocation of strata which occur on them. 



Origin and Occurrence of Mountain Pedestals. — Some years ago, with 

 these points in mind, I studied the Italian peninsula, making, partly in the 

 field and partly by maps and reports, several sections across its surface 

 from the Adriatic on the east to the Tyrrhenian sea on the west. I chose 

 this peninsula for the reason that its geology is fairly well known, as is 

 also the shape of the sea-bottom on either side of the land. An inspec- 

 tion of this area convinced me that, along with the folding and other 

 compressive phenomena which here produced the mountain-axes of this 

 district, there has been a progressive uprising, in a massive way, of the 

 deposits which form the outer part of the earth, and that this basilar 

 uplift extends for a considerable distance on either side of the much 

 distorted areas. Such a foundation uplift or pedestal occurring in moun- 

 tain-building has long been noted, but its full and true import seems not 

 to have been recognized. It apj^ears clearly to indicate that during the 

 process of folding and the concomitant faulting there is a widespread 

 movement of the underlying rocks toward the seat of disruption. This 

 material, which is set in motion by the orogenic forces, not only serves 

 to underpin or support the arches of the strata, but accumulates in the 

 neighboring subjacent regions in such amount as to uplift the surface of 

 the earth in the form of a broad belt, on which, at the end of the process, 

 the mountains lie as sharp ridges. 



It is obvious that in many cases the pedestal or highlands, where the 

 strata are not distorted, which lie on one or both sides of many mountain- 

 ranges is often in part due to the thickening of strata, brought about by 

 the supply of sediments that the elevations have afforded to the seas at 

 their feet. Making allowance for this source of error, I think it is still 

 evident that the pedestal feature is a normal element in mountain-growth. 



On the continent of North America we discern three great systems of 

 elevation, two of which, the Cordilleras and the Appalachian, show the 

 more or less degraded remains of the basilar uplifts, which were here 

 produced during the development of the axes. The central trough of 

 this land, so far as it is occupied by the Mississippi valley, is substan- 

 tially composed of the slopes which lead down from the great systems 

 on the east and west. The Laurentian system shows less of this pedestal 

 feature, probably because it was to a great extent destro37'ed during the 

 ages which elapsed between its formation and the deposition of the strata 

 w^hich we trace into contact with its ancient ridges. 



