354 HISTORY OF GEOLOGY AND PALAEONTOLOGY. 



Elie de Beaumont was the first to point out the contrast 

 between widespread or normal metamorphism and the contact 

 metamorphism which was limited to smaller zones of rock, and 

 especially to the contiguous parts of eruptive and sedimentary 

 rocks. Daubree afterwards applied the term of regional meta- 

 morphism to distinguish these processes which had acted 

 throughout vast regions of the crust and altered thick forma- 

 tions of rock. 



One of the extreme Neptunists was Johann Fuchs, who 

 explained the crystalline schists, gneiss, granitic and porphy- 

 ritic rocks as segregation products from a watery or pasty 

 material. The American geologist, Professor Dana, in 1843 

 thought that the Huttonian doctrine did not attach sufficient 

 importance to the agency of heated water in effecting rock- 

 metamorphism. He compared gneiss with volcanic tuffs, and 

 held the opinion that during invasions of granitic magma into 

 the upper zones of the crust a granitic ash also escaped, and 

 under the influence of superheated water became caked and 

 cemented into the form of gneissose and schistose rocks. 

 J. Bischof, in several papers published between the years 1847 

 and 1854, agreed with Keilhau in assuming that the oldest 

 sediments were for a long time supersaturated with water, and 

 that chemical changes had slowly altered their constitution, 

 converting argillaceous sediments first into clay-slate, and by 

 continuance of the chemical processes into micaceous schists. 



Scheerer contributed in 1847 a suggestive paper on the 

 origin of gneiss, in which he took the standpoint that it might 

 be produced in various ways and from various rocks. He 

 explained the gneiss of the Erz mountains as a rock that had 

 been metamorphosed from sedimentary strata in situ, whereas 

 the red gneiss during the time of its metamorphism had under- 

 gone flow movements comparable to those of an eruptive 

 magma. Again, in many cases gneiss was a fundamental 

 Archaean rock representing a portion of the primaeval crust of 

 the earth. Cotta also thought that most gneiss had formed 

 part of the original crust, but he regarded the crystalline 

 schists as the culminating result of a process of metamorphism 

 undergone by all sedimentary rocks which had already been, 

 or were now in process of being, covered by a thick mantle of 

 younger deposits. The change, he thought, had been effected 

 by heat and pressure, possibly in combination with water; and 

 although the crystalline schists were in many places now ex- 



