70 SECTIONAL ADDRESSES. 
About 1880 the geologists of the United States revealed phenomena in 
their Western Mountains which showed that the yardstick that had been 
proved reliable in North-Western Europe and the Atlantic States of 
America, was not applicable everywhere. The United States Geological 
Survey had in 1870 published only two of its Annual Reports and a 
preliminary report on Colorado ; it issued the first of its Bulletins in 1874 
and Gilbert’s Monograph on the Henry Mountains in 1877: its great 
influence began about 1880. Nevertheless, despite the powerful stimulus 
of North America on geological thought in the third quarter of the past 
century the individual influence that seems to me to have been most 
profound was that of E. Suess. 
The last quarter-century is still too near for reliable appreciation of 
its achievements; but among the fundamental advances have been 
those revealing the structure of the inner earth, and especially the inter- 
pretation of earthquakes, in which the pioneer was a devoted adherent of 
the Association, John Milne. The recent study of ore-deposits confirms 
the evidence from earthquakes that the core of the earth is surrounded by 
concentric shells and that the metallic ores arise from the shell below the 
plutonic rocks as gases and solutions. The distribution of ores, if those 
of any particular metal be considered in reference to their age, is also along 
bands, due to ruptures in the crust through which the volatile and liquid 
ore-transporting agents escape from the metallic barysphere. The geology 
of mineral fields and the extension to most of the younger mountain ranges 
of the world of the thrustplanes which, though early recognised in mining 
operations, were first demonstrated in stratigraphy by Lapworth and the 
British Geological Survey in the North-West of Scotland, have proved 
that fold-mountains are formed along belts of compression. The intense 
folding of the crust which at first was world-wide has been confined in 
later times to narrow belts—showing that the accommodation of a 
thickening rigid crust to a contracting internal mass has been the 
dominant influence on geological evolution. 
