VERTICAL UPLIFT OF MOUNTAIN RANGES 321 



Lower Carboniferous, strata. Therefore there was depression after the 

 folding of the Silurian strata, and later elevation without folding. 

 Nearly the same history applies to the Blue Mountains of southeastern 

 Australia. Nearly horizontal Carboniferous and other deposits, some of 

 them marine, rest unconformably on strongly folded Devonian. The 

 mountains are separated from the coastal area by a great monoclinal 

 flexure. Their topography resembles the mesa topography of the western 

 United States. 



If we approach the Himalaya Mountains from the south we pass over 

 the great alluvial plain of the Ganges, consisting of fluviatile deposits 

 horizontally stratified. This plain is limited on the north by the sub- 

 Himalaya or Siwalik Mountains, about 3,000 feet high, the boundary 

 being supposed to be a fault. The Siwaliks consist of the same kind of 

 material as the Ganges plain and evidently once formed a part of it. The 

 strata, of middle and late Tertiary age, have been considerably folded 

 and faulted. Beyond the Siwaliks, and separated from them by the great 

 and nearly vertical main boundary fault, rise the Lesser Himalayas to a 

 height of 12,000 to 15,000 feet. They consist of crystalline and stratified 

 rocks, apparently very old, and are much more folded and crushed and 

 are greatly metamorphosed. Beyond these again a sudden rise brings us 

 to the culminating Central Himalayas, which are largely crystalline and 

 appear to have suffered still more crushing. The Himalayas, therefore, 

 rise from the plain by one short and two large steps. Many faults, prin- 

 cipally parallel with the trend of the range, occur over the whole range ; 

 nearly all are reverse faults and are very steep. They seem to have been 

 formed in roughly progressive order from the central region to the south- 

 ern border. All parts of the range have suffered compression, strong in 

 the heart of the range and considerably less in the weak Siwaliks. 



The intense foldings and dislocations of the Himalayas could not have 

 been produced in the present position of the rocks; for not only is there 

 no sufficient buttress on the south to sustain the necessary pressure, but 

 the folding of the strong rocks of the Lesser Himalayas suddenly becomes 

 much less as we cross the main boundary fault into the weak strata of 

 the Siwaliks. The pressure could not have diminished suddenly at a 

 fault surface, nor could the compression of the strong rocks be less than 

 that of the weaker under the same pressure. We conclude that the fold- 

 ings and some faulting of the Himalayan rocks took place before their 

 elevation, when they were continuous with the bedrock of the Ganges 

 basin, and that the folding continues under that basin, probably dying 

 out to the south, much as the Appalachian folds die out to the west. 

 Hayden, from an examination of the Himalayan rocks themselves, con- 



XXI— Bull. Geol. Soc. Am., Vol. 33, 1921 



