356 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



the sediments above and assumed the form of eruptive massive 

 protrusions. 



The microscopic examination of micaceous schist led Sorby 

 (1856) to the assumption that it had originally been a shale 

 and had been altered probably by means of water, a high 

 temperature, and crust pressure. He regarded the foliated 

 structure as a result of mechanical pressure. Hitchcock, in 

 1861, also emphasised the action of mechanical strains. 



Sir William Logan's discovery, in 1867, of the thick series 

 of gneisses and schists forming the floor of the sedimentary 

 succession in Labrador and Canada, gave for a time additional 

 support to the view of the Archaean age of all metamorphic 

 rocks ; but every year stratigraphical researches were bringing 

 new facts to light which could not harmonise with this simpler 

 view of one primaeval epoch of formation for the crystalline 

 foliated rocks. 



Zirkel, in 1866, made a complete resume of the literature on 

 the subject of the metamorphic gneiss, and after a careful 

 criticism of the facts and arguments, concluded that there is 

 probably an original gneiss and a metamorphic gneiss. Water 

 and the plastic magma have participated in the formation of 

 the former ; it formed the first solid crust, and could, under 

 certain circumstances, especially in the immediate vicinity of 

 granite, partake of its eruptive character. Gneiss has either 

 taken origin from shales and grits by contact metamorphism 

 in the presence of heated water, or has arisen from the sub- 

 terranean transformation of sedimentary strata by means of 

 some simple processes of water-permeation, which have so far 

 eluded discovery. Zirkel also explains the origin of granulite 

 and the other crystalline schists upon principles of water- 

 permeation, but he regards micaceous and chloritic schists 

 and phyllites as metamorphosed sediments. 



Lossen initiated a new departure in the investigation of 

 the metamorphic group, in so far as he succeeded in impressing 

 geologists with the high value of accurate field investigations 

 in assisting the solution of some of the intricate problems 

 of metamorphism. During his examination of the Taunus 

 mountains (1867), Lossen formed the opinion that most of 

 the crystalline schists had originated as sedimentary strata 

 containing a large amount of interstitial water, and had been 

 cleaved and altered by the action of strong dynamic pressures 

 during the mountain-making movements. Gneiss and mica 



