THEORETICAL CONSIDERATIONS 403 



can be laid only through observations of the intensity of gravity which 

 actually exists in different sections and consideration of the qualifying 

 effects of tangential pressure and igneous activity. Nevertheless the 

 speculation is worthy of being entertained. The observed vertical dis- 

 placements of negative elements with reference to positive elements are 

 equal to or of the same order of magnitude as oceanic depths ; the masses 

 involved are some of them of subcontinental dimensions; the rate of 

 subsidence or elevation has been gradual in general, though periodically 

 accelerated, and for a given element the movement has been in the same 

 sense positive or negative during long geologic periods. 



Hence I postulate as a basis of a theory of continental structure that 

 the continent consists of elements which differ in density and which have 

 heen controlled in certain movements of elevation or subsidence hy the 

 differences of density existing among them. 



Effects op tangential Pressure 



In the conclusion just stated, the effects of isostatic adjustment are 

 qualified by restriction to certain movements of elevation or subsidence. 

 Certain other movements of elevation, and possibly also of depression, 

 are attributable to tangential pressure. Further, it is thought probable 

 that the two effective conditions (deficient density and horizontal thrust) 

 may combine to cause a given elevation. And, finally, it is recognized that 

 as a result of tangential" pressure a region which had previously pos- 

 sessed the character of a negative element and suffered depression may 

 become a region liable to uplift. 



These relations may best be illustrated by reference to the history of 

 Appalachia and the adjacent negative elements on the west. The pre- 

 Cambrian movements of Appalachia, so far as we know them, resulted 

 in profound erosion and were movements attributable to isostatic eleva- 

 tion. Whether the negative elements were then effectively differentiated 

 or not, we do not know. During the Paleozoic the positive movements 

 continued to be of an isostatic type and there were negative displace- 

 ments of the same kind. Only in the Devonian was there an exception- 

 ally energetic orogenic effect which might be attributed to tangential 

 thrust, and, restricted as this was to a district east of Pennsylvania and 

 New York, the uplift may have belonged to the New England province, 

 which appears to have had a distinct dynamic history, as will appear in 

 the further discussion. 



The Appalachian revolution greatly modified the previously existing 

 conditions. Appalachia, the positive element, apparently lost a consider- 



