STRENGTH OF THE EARTHS CRUST. 25 



If this hypothesis is strictly true, then there should he no single mountain mass 

 and no single valley, due purely to the local addition or subtraction of material, 

 having a greater volume than 600 cubic miles. At least four kinds of mountains and 

 valleys are due simply to the addition and subtraction of material : (1) mountains of 

 extravasation (such as volcanic cones) beneath which the pre-existent terranes lie 

 undisturbed; (2) mountains of circumdenudation, produced by the removal of sur- 

 rounding material ; (3) mountains produced by extravasation and circumdenudation ; 

 (4) valleys of erosion, unaccompanied by phenomena of displacement. 



A large number of such mountains and valleys exist, and some of the largest 

 occurring in the United States have been mapped in contours by the U. S. Geological 

 Survey 3 so that their volumes can be computed readily. 



San Francisco Mt., in Arizona, a result of extravasation, has a volume of 40 cubic 

 miles. 



Mt. Shasta, probably due to extravasation only, has a volume of 80 cubic miles. 



The Tavaputs Plateau, or Roan Mt., lying on the borders of Utah and Colorado, 

 and produced by circumdenudation, has a volume of 700 cubic miles. 



Mt. Taylor, and the Taylor Plateau, in New Mexico, resulting from extravasation 

 and circumdenudation, have jointly a volume of 190 cubic miles. 



The Henry Mts., resulting from volcanic intrusion and circumdenudation, have a 

 volume of 230 cubic miles. 



The Sierra La Sal, a mountain group of the same type, has a volume of 250 cubic 

 miles. 



The deeper portion of the Grand Canon of the Colorado, from the mouth of the 

 Little Colorado to the mouth of Kanab Creek, is due to the removal of 350 cubic 

 miles of rock. 



The Tavaputs Plateau slightly exceeds the hypothetic limit; the other illustrations 

 fall within it. 



In view of the phenomena cited, and of the considerations and comparisons ad- 

 duced, it is believed that the following theorem or working hypothesis is worthy of 

 consideration and of comparison with additional facts : Mountains, mountain ranges, 

 and valleys of magnitude equivalent to mountains, exist generally in virtue of the 

 rigidity of the earth's crust ; continents, continental plateaus, and oceanic basins exist 

 in virtue of isostatic equilibrium in a crust heterogeneous as to density. 



Professor A. Winchell : It strikes me that Mr. Gilbert's position is pretty nearly 

 correct. I thought when he commenced that he was likely to discount the old doctrine 

 of surface inequalities existing by virtue of rigidity in the crust. I found in the end, 

 however, that he recognizes the validity of the old, generally received theory that 

 the height of mountains depends upon the rigidity of the crust. He recognizes that, 

 as I understand. I think that view, connected with the earlier suggestions of Sir 

 John Herschel and some of the later determinations of his son, is one so well estab- 

 lished that it would require very unquestionable facts in the line of those Mr. Gilbert 

 has furnished to overthrow the conclusions in which geologists generally are resting. 

 It is obvious, also, that there is truth in the suggestion that inequalities depend partly 

 for their existence upon differences in the density of the material, and so far as Mr. 

 Gilbert has used that principle in its application to the continental saliences of the 

 earth's crust, I do not know but he is entirely within the limits of probability. Not- 

 withstanding my adherence to the old doctrine, I am ready to admit there are certain 



IV— Bull. Geol. Soc. Am., Vol. 1, 1889. 



