Geology and Magnetism. 509 



It may be considered strange that such an universal structure 

 has escaped attention, and that it has not ere this been discovered. 

 The isolated facts have been long known, but not properly used, 

 and the laminated structure has been, and continues to be, con- 

 founded with those planes resulting from mechanical deposition. 

 As we have already noted, the granitic gneiss and the schistose, 

 which are commonly represented in geological sections as sedimen- 

 tary beds resting on one another, are the result of a crystalline ac- 

 tion modifying the granitic mass in the direction of the lamination, 

 which structure is generally formed in a more or less vertical posi- 

 tion as commonly seen when not disturbed. 



" Since the year 1792," says Humboldt in his treatise on Rocks, 

 " I have been attentive to the parallelism of beds. Residing on 

 mountains of stratified rocks, where this phenomenon is constant, 

 examining the direction and dip of primitive and transition beds, 

 from the coast of Genoa across the chain of the Bochetta, the plains 

 of Lombardy, the Alps of St. Gothard, the table-land of Suabia, the 

 mountains of Baireuth, and the plains of Northern Germany, I have 

 been struck, if not with the constancy, at least with the extreme fre- 

 quency of the directions from south-west to north-east. This in- 

 quiry, which I thought would lead naturalists to the discovery of 

 a great law of nature, at that time interested me so much, that 

 it became one of the principal reasons for my voyage to the equator. 

 When 1 arrived on the coast of Venezuela and passed over the lofty 

 littoral chain and the mountains of granite-gneiss that stretch from 

 the Lower Oronoco to the basin of the Rio Negro and the Amazon, 

 I recognized again the most surprising parallelism in the direction 

 of the beds ; that direction was still north-east." 



Unfortunately, in consequence of Werner's theory, Humboldt 

 wrote the above under the impression that the gneiss and schistose 

 rocks were similar to sedimentary beds ; hence he confounds the 

 lamination of the former with the divisional planes of the latter. 



" When we examine," says M. Boue, " with a compass the posi- 

 tion of mineral masses in Scotland, and endeavour to stop at general 

 facts, we perceive that the direction of the beds is constant, and 

 corresponds with that of the chains from south-west to north-east, 

 but that the dip varies according to local circumstances." 



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