Editing Formation of rocks (section)

== Rock formation ==

=== 19th-century efforts to synthesize rocks === <!-- this section appears to be substantially from the 1911 edition of Encyclopedia Britanica -->

The [[Chemical synthesis|synthetic]] investigation of rocks proceeds by experimental work that attempts to reproduce different rock types and to elucidate their origins and structures. In many cases no experiment is necessary. Every stage in the origin of clays, sands and gravels can be seen in process around us, but where these have been converted into coherent shales, sandstone and conglomerates, and still more where they have experienced some degree of metamorphism, there are many obscure points about their history upon which experiment may yet throw light. Attempts have been made to reproduce igneous rocks, by fusion of mixtures of crushed minerals or of chemicals in specially contrived furnaces. The earliest researches of this sort are those of [[Barthélemy Faujas de Saint-Fond|Faujas St Fond]] and of [[Horace-Bénédict de Saussure|de Saussure]], but Sir [[Sir James Hall, 4th Baronet|James Hall]] really laid the foundations of this branch of petrology. He showed (1798) that the whinstones ([[diabase]]s) of Edinburgh were fusible and if rapidly cooled yielded black vitreous masses closely resembling natural pitchstones and [[obsidian]]s. If cooled more slowly they consolidated as crystalline rocks not unlike the whinstones themselves and containing [[olivine]], [[augite]] and [[feldspar]] (the essential minerals of these rocks).<ref name=EB1911>{{EB1911|wstitle=Petrology |volume=21 |pages=326–327 |inline=1 |first=John Smith |last=Flett}}</ref>

Many years later [[Gabriel Auguste Daubrée|Daubrée]], [[Achille Ernest Oscar Joseph Delesse|Delesse]] and others carried on similar experiments, but the first notable advance was made in 1878, when [[Ferdinand André Fouqué|Fouqué]] and [[Auguste Michel-Lévy|Lévy]] began their researches. They succeeded in producing such rocks as porphyrite, [[leucite]]-[[tephrite]], [[basalt]] and dolerite, and obtained also various structural modifications well known in igneous rocks, e.g. the [[porphyritic]] and the [[Poikilitic#Ophitic|ophitic]]. Incidentally, they showed that while many basic rocks (basalts, etc.) could be perfectly imitated in the laboratory, the acid rocks could not, and advanced the explanation that for the crystallization of the latter the gases never absent in natural rock magmas were indispensable mineralizing agents. It has subsequently been proved that steam, or such volatile substances as certain borates, molybdates, chlorides, fluorides, assist in the formation of [[orthoclase]], [[quartz]] and [[mica]] (the minerals of [[granite]]). Sir James Hall also made the first contribution to the experimental study of metamorphic rocks by converting [[chalk]] into [[marble]] by heating it in a closed gun-barrel, which prevented the escape of the [[carbonic acid]] at high temperatures. In 1901 [[Frank Dawson Adams|Adams]] and [[John Thomas Nicholson|Nicholson]] carried this a stage further by subjecting marble to great pressures in hydraulic presses and have shown how the foliated structures, frequent in natural marble may be produced artificially.<ref name=EB1911/>