1. INTRODUCTORY 1
The Problem 1
Vocabulary 5
Discontinuities in the Earth 16
Calculated Internal Densities 21
Figure of the Earth; Geoid, Ellipsoid, Spheroid 24
2. DEVELOPMENT OF THE IDEA OF ISOSTASY 36
Bouguer to Petit 36
Pr 38
Airy 42
Hall 50
Jamieson 50
Dutton 51
Gilbert 53
Helmert ; Schweydar 54
Suggestions Regarding Isostatic Compensation 56
3. TESTING ISOSTASY WITH THE PLUMB-LINE 65
Deflections of the Vertical 65
Hayford's First Investigation 67
Topographic Deflections 70
Construction of the Geoid 78
Recognition of Isostasy 81
Effect of Isostatic Compensation on the Direction of the Vertical 87
Statistical Tests of Conclusions 94
Degree of Isostatic Compensation 99
Amended Ellipsoid of Reference 102
Hayford's Second Investigation 102
Conclusions Regarding the Dimensions of Uncompensated Loads 104
4. MEASUREMENT OF GRAVITY; COMPARISON OF INTENSITIES 106
Gravitation 106
Relation of Gravity to Latitude 109
Measurement of Gravity 111
Reduction of Values of Gravity 114
Free-air Reduction 114
Bouguer Reduction 116
Isostatic Reductions 120
Indirect Reduction 126
Clearing-house for Systematic Record of Anomalies 129
5. GRAVIMETRIC TESTS OF ISOSTASY IN THE UNITED STATES, BY HAYFORD AND BOWIE 131
Introduction 131
Assumed Conditions for Isostatic Compensation 132
Theoretical Gravity at a Station 139
Correction for Elevation 139
Correction for the Effects of Topography and Compensation 140
Specimen Isostatic Anomalies 149
Comparison with the Corresponding Free-air and Bouguer Anomalies 149
Local Versus Regional Compensation 152
Relation of Anomaly to Varying Depth of Compensation 154
Anomalies in Terms of Masses 154
General Conclusions 156
6. LATER GRAVIMETRIC TESTS OF ISOSTASY IN NORTH AMERICA 159
Bowie Report of 1912 159
Bowie Report of 1917 161
Bowie Report of 1924 174
United States Values of Gravity Reported Since 1924 182
Putnam's Discussion of United States Data 184
Gilbert's Estimate of Uncompensated Loads 187
Heiskanen on Gravity in the United States 187
Gravity in Canada 190
Concluding Remarks 191
7. TESTING ISOSTASY IN EUROPE 196
Introduction 196
Gravity in Alps and Apennines 197
Heiskanen's Studies 205
Caucasus 205
Norway 208
Gravity in Finland 210
Gravity Measurements in Cyprus 211
Additional European Data 213
Review and Forecast 215
8. TESTING ISOSTASY IN AFRICA AND ASIA 218
Earlier Work in East Africa 218
Bullard on Gravity in East Africa 219
Gravity in Siberia 224
Conditions in India 224
The Geoid in India 225
Gravity Anomalies in India 236
Observations in Turkestan 247
Gravity Anomalies in Japan 248
Some Leading Results 250
9. TESTING ISOSTASY AT SEA 252
Introduction 252
Observations of Hansen, Schi?tz, and Hecker 253
Measurement of Gravity at Sea, by Vening Meinesz 256
Mean Gravity Anomalies Over the Open Ocean 259
Gravity Anomalies Over Mediterranean Seas 264
Strip of Negative Anomalies, East Indies 269
Negative Strip of the West Indies 285
Anomalies Over Ocean Deeps Facing Mountainarcs 290
Gravity Profiles off Shores 294
Gravity Anomalies in the Vicinity of Volcanic Islands 298
Gravity Anomalies at Stations Near Great Deltas 303
Conclusions 305
10. NATURE'S EXPERIMENTS WITH ICECAPS 309
Introduction 309
Jamieson Hypothesis 310
Warping of Fennoscandia, Past and Present 312
Objections to Jamieson's Isostatic Hypothesis 316
Recoil of the Glaciated Tract of Northeastern North America 326
Nansen's Evidence from the Strandflat 330
Summary 334
11. RETROSPECT 337
Introduction 337
Meaning of the Word "Isostasy" 338
Proofs of Close Approach of the Earth to Ideal Isostasy 342
Relation of Gravity Anomaly to Spheroid of Reference 344
Need of Great Extension of the Gravitational Survey 345
Effect of Errors of Mensuration and Computation 348
Effect of Changing the Assumed Type of Isostatic Compensation 349
Effect of Changing the Assumed Depth of Compensation 350
Effect of Local Abnormalities of Density of Rock 351
How Permanent are Areas of One-sign Anomaly? 351
12. STRENGTH OF THE EARTH-SHELLS 353
Introduction 353
Stresses Caused by Departures from Isostasy 355
Dimensions of Stable Areas of One-sign Anomaly 361
Anomaly Areas in the United States 362
Mechanical Conditions in Peninsular India 364
Anomaly Field Along the Pacific Coast of North America 369
Anomaly Field of the Eastern Atlantic and Adjacent Land 371
Anomaly Field of the Western Pacific 373
Summary 373
Anomaly Fields of the Mediterranean Seas 373
Anomaly Fields of the Open-ocean "Deeps" 376
Anomaly Fields Surrounding Volcanic Islands 379
Comparison of Loads on Continental and Oceanic Sectors; Strength of the Lithosphere 380
Experiments on the Relation of High Confining Pressure to the Strength of Rock 384
The Unstable Glaciated Tracts 385
Comparison of Conditions in India and Fennoscandia 390
Horizontal Variation of Thickness of the Lithosphere 393
Strength of the Lithosphere at Earlier Geological Epochs 396
The Asthenosphere and Theories of Diastrophism and Petrogenesis 398
The Asthenosphere and Deep-focus Earthquakes 400
The Asthenosphere and Warped Peneplains 407
Strength of the Mesospheric Shell 408
Conclusions 410
INDEX 419