PETROGRAPHY. 357 



schists had been, he thought, parts of the original granitic 

 crust of consolidation, which had been similarly converted 

 by pressure-metamorphism into banded, foliated, and cleaved 

 rock-facies. Lessen subsequently examined and mapped the 

 Harz mountains geologically, and found further confirmation 

 of his theory of "dislocation metamorphism." He demon- 

 strated in the Harz mountains that the same rocks which 

 extended over wide regions as ordinary shaly sediments could 

 be traced into a zone of crust disturbance, where they became 

 crystalline and schistose, and were split by planes of cleavage 

 superinduced upon the rock-strata at various angles with the 

 planes of stratification. Although Lossen's work threw a new 

 interest into phenomena of cleavage, the presence of cleavage- 

 planes had long been known in certain rocks. As far back 

 as the eighteenth century, Lasius and Voigt had drawn at- 

 tention to the difference between the planes of stratification 

 and planes of cleavage, but could not find any explanation. 

 Sedgwick (1822 and 1835) suggested that the cleavage of rocks 

 might be due to the action of polar forces along a definite 

 direction, causing orientation of crystals in that direction. 

 J. Phillips, in 1843, at a meeting of the British Association, 

 pointed out the deformation of fossils in cleaved rocks, and 

 thought cleavage was the result of a slow creep of the minute 

 rock particles in a definite direction. An important observa- 

 tion was made by the brothers Rogers, who showed, in 1837, 

 that the cleavage-planes in the Alleghany mountains extended 

 parallel with the main axis of upheaval of this mountain 

 system, but in explanation they accepted Sedgwick's theory 

 of polar attraction. 



Almost simultaneously, the action of lateral pressure was 

 suggested by two observers: in 1846 by Baur, an overseer 

 of mines in Eschweiler, who explained the cleavage of the 

 greywackes in the Rhine Province by this means; and 

 in 1847 by D. Sharpe. Sorby in 1853 made pressure 

 experiments, and succeeded in reproducing cleavage arti- 

 ficially in different kinds of rock. His results were sup- 

 ported by the later experiments of Tyndall (1856) and 

 Daubree (1861). 



When, therefore, Lessen from his actual field observations 

 drew the important conclusion that crust disturbance had been 

 the chief agent in effecting cleavage metamorphism, he was in 

 a position to refer to the confirmatory evidence in favour of 



