458 PROF. H. D. ROGERS ON THE LAWS OF STRUCTURE 



Views of Geologists concerning Cleavage and Foliation. 



Professor Sedgwick, as early as 1822, discovered and subsequently publicly 

 taught the true nature of slaty cleavage, distinguishing it from joints, and show- 

 ing it to be a tendency to separation in perfectly parallel planes, which are irre- 

 spective of the bedding. He ascertained that the slaty cleavage is usually con- 

 fined to the finer-grained rocks, alternating coarser beds possessing it very im- 

 perfectly, and laid it down as a rule, that the strike of the cleavage is nearly 

 coincident with the strike of the beds He referred it to crystalline or polar forces 

 acting simultaneously and somewhat uniformly in giving directions. Subsequently, 

 Professor Sedgwick in 1835,* after many additional observations on the modifica- 

 tions of slaty cleavage, showed that the rule admitted of many limitations, which 

 the geologist is compelled to notice in working out the structure of complicated 

 districts. In a recent publication, his " Synopsis of the Classification of the 

 British Palaeozoic Rocks," he shows conclusively, that the cleavage structure is 

 " the compound effect of all the crystalline forces acting on the mass, and that it 

 cannot be due to a mechanical action." In this work, he also mentions the im- 

 portant fact of the existence frequently of a second cleavage plane, generally in- 

 clined at a great angle to the primary. 



Sir J. Herschel has suggested that the rocks possessing cleavage may have been 

 so heated as to allow a commencement of crystallization, or heated to a point at 

 which the particles may have begun to move among themselves, or on their own 

 axes ; surmising that some general law has determined the positions on which 

 the particles have rested on cooling, and that this position has had some relation 

 to the direction in which the heat escapes, j 



Professor Phillips % has shown, that in some slaty rocks, fossil shells and tri- 

 lobites have been much distorted by cleavage ; and he imputes this to a creeping 

 movement of the particles of the rocks along the cleavage planes. This displace- 

 ment, uniform over the same tract of country, he states to be as much as a 

 quarter or even half an inch. Hard shells are not thus affected, but only the 

 thin ones. Professor Phillips, in 1843, stated that the cleavage planes of the 

 slate rocks of North Wales are cleavages parallel to the main direction of the 

 great anticlinal axes. 



Mr Daniel Sharpe conceives that the present distorted form of the shells in 

 certain slates, has been produced by a compression in a direction perpendicular 

 to the planes of cleavage, and an expansion in the direction of the cleavage dip.$ 



He conceives that the planes of cleavage range vertically along certain 

 lines or belts, and dip towards those lines on each side of them ; those nearest 

 the central vertical belts at high angles, the angles gradually diminishing as 



* Gcol. Trans., 2d Series, iii., p. 461. t Lyell's Manual, p. 610. 



J Report British Association, Cork, 1843. § Quarterly Jour. Geol., viii., p. 87. 



