THE STRENGTH OF THE EARTH'S CRUST 29 



regarded as a continuation of the same subject, an added contri- 

 bution in the large field of diastrophism and the formative pro- 

 cesses, following out certain of its ramifications. 



A somewhat general survey is given here of the problern of the 

 strength of the crust, beginning with the lines of evidence which 

 bear upon it and following out to some degree the conclusions drawn 

 from it. It has in this way been cast into the form of those articles 

 published by the Journal of Geology from time to time, under the 

 caption of "Studies for Students." 



PART I. GEOLOGIC TESTS OF THE LIMITS OF STRENGTH 

 INTRODUCTION AND SUMMARY 



The capacity of the outer crust to resist vertical stresses is an 

 important field in the theory of dynamical and structural geology. 

 On the one hand, it is known that the larger segments, those of 

 continental and oceanic proportions, rest to a large degree in 

 isostatic equilibrium, the subcrust of the continental areas being 

 lighter than that of the oceanic areas in proportion to the regional 

 elevation. On the other hand, the minor features, those which 

 enter into the composition of the landscape, are known to have 

 been sculptured by external forces and are to be explained there- 

 fore as sustained by reason of the rigidity of the crust. 



Between these two extremes in magnitude of terrestrial rehef 

 lie mountain ranges, plateaus, and basins; made in part by tangential 

 forces, modified by erosion and sedimentation. To what extent 

 can these constructional and destructional forces work in oppo- 

 sition to those other forces which by producing vertical movement 

 make for isostatic equilibrium ? The method of attack is from 

 two directions. The geologist examines the structures imposed by 

 tangential forces, the mountains built by igneous extrusion, the 

 surfaces made by erosion, the strata consequent upon sedimenta- 

 tion. From them he may determine the amount of strain which 

 the crust can endure before periodic movements occur in the direc- 

 tion of relief from strain. The geodesist, by means of the plumb- 

 line and pendulum, determines the subcrustal densities and notes 

 the degree to which these are balanced against the reHef, pointing 



