THE author's summary 261 



Summary 



In this generalized view of the cross-section of the Appalachian clias- 

 trophic effects in southern New England, the middle parts exhibit no 

 clear signs of engulfment between overturned and outthrust marginal 

 folds, such as Holmqnist invoked in his ideal sketch of a collapsed geo- 

 .syncline. On the contrary, the middle portion (New Brunswick geanti- 

 cline of Schuchert) stands relatively high in the closing stages of the two 

 geosynclines of deposition (Saint Lawrence and Acadian) on either side 

 of it and was relatively stable and unyielding during the subsequent 

 phase of strong compression and collapse, its movements, if anything, 

 being then upward rather than downward. 



As I now see it, the potent factor in mountain-building and vulcanism 

 lies in the depths beneath disturbed belts of mountain systems, wherein 

 crust and magma move under the control of a vertical force, of which 

 ever-present gravity and changes of temperature, however caused, are 

 components, having diverse effects at the surface of the earth under the 

 secondary influences of deposition and erosion. An upstanding belt of 

 old rocks, flanked on one or both sides by detrital troughs of now col- 

 lapsed strata more or less overturned from the median region, and here 

 and there invaded by metamorphosing igneous intrusions, shown in the 

 cross-section of a mountain system of folded and locally overthrust strata 

 and basal crystalline schists, strengthens the writer's confidence in the 

 doctrine of isostasy as a statement of the working conditions under which 

 mountain-building w^ent forward in the Appalachian region after the 

 initial geanticline and geos3^nclines were produced to set in motion a 

 transfer of load by erosion and deposition. Once it is recognized that 

 the historical "Appalachian geo&yncline^' of Dana, first recognized by 

 James Hall of Alban}^ is a lateral affair in a broad mountain-built zone, 

 the almost complete lack of igneous intrusives in that portion of the 

 section, does not exclude the role of magmatic movements in the problem 

 of Appalachian mountain-building in its broader sense, and viewed in 

 its full scope the Appalachians find their counterparts in the great 

 Cordilleras of the Pacific region. 



