5\2 Geology and Magnetism. 



almost geometrical parallelism, while they pass through contorted 

 strata of hard slate, obviously of sedimentary origin. Crystalline 

 forces have re-arranged whole mountain masses of them, producing 

 a beautiful crystalline cleavage, passing alike through all the strata. 

 And again, through all this region, whatever be the contortions 

 of the rocks, the planes of cleavage pass on, generally without de- 

 viation, running in parallel lines from one end to the other, and 

 inclining at a great angle to the west. Without considering 

 the crystalline flakes along the planes of cleavage, which prove 

 that crystalline action has modified the whole mass, we may affirm 

 that no retreat of parts, no contraction in dimensions, in passing 

 to a solid state, can explain such phenomena as these. They 

 appear to me only resolvable on the supposition that crystalline or 

 polar forces acted on the whole mass in given directions and with 

 adequate power." 



In the Geological Report of Cornwall and Devon, Sir H. de la 

 Beche remarks : — 



" When we regard the prevalence of the great divisional planes 

 in particular directions crossed by other nearly at right angles to 

 them, producing solids to a certain extent symmetrical, and consi- 

 der the mineral modifications which the sedimentary beds have 

 generally undergone since they were deposited, we are led to sus- 

 pect not only that the lamination planes, commonly termed cleavage, 

 are, as has been supposed by some authors, due to polar forces, 

 but also that the great divisional planes have been equally caused 

 by them, as has been considered probable by others." " Although 

 the direction of the present magnetic meridian in the district may 

 be merely temporary, and the proximation of so many great division- 

 al planes to it therefore accidental, still their great prevalence, both 

 in the igneous (crystalline) rocks and sedimentary deposits, in that 

 direction, leads us to suppose that polar forces may have consider- 

 ably governed the arangements of the component matter of the 

 rocks they traverse during consolidation. If we require a con- 

 stant tendency of such polar forces to arrange the component 

 matter of rocks during consolidation in given areas, we can the 

 more readily account for the frequency of nearly similar directions 

 in the great divisional planes of rocks of different ages." 



