67 



squeezed into folds by the vertical pressure of S, sand, extra 

 weight being applied, 



211. Specimens illustrating the production by compres- 

 sion of natural and artificial slaty cleavage. 



1876, Lent by Dr. E. G. Sorby, F.R.8. 



These specimens are of historical interest, as illustrating the 

 Presidential Address to the Geological Society by Dr. Sorby in 

 1880. See Q.J.G.S., Vol. XXXVI Dr. Sorby's description of 

 these specimens is : 



" Specimens of slaty rocks, showing by various facts that they 

 have been greatly compressed in a line perpendicular to the 

 cleavage. 



" Pipe-clay with portions of blue paper, and also with iron 

 scales, being the results of the first experiments made to show 

 that a structure like that which causes the cleavage in slates 

 can be artificially produced by pressure. 



<l Artificial cleavage in compressed flaky graphite, being as per- 

 fect as in any slaty rock." 



These specimens consist of a mass of sandstone in which there 

 is a spherical green patch ; two blocks of purple Penrhyn slate 

 in which a similar green patch is compressed into a spheroid, 

 and a third block in which there is an irregular green band. 

 There is also a piece of fossiliferous Devonian slate in which 

 fossil Brachiopods, which were originally symmetrical, have been 

 drawn out and. elongated in the plane of cleavage. 



There are four specimens of pipe-clay with specks of blue 

 paper, one of which is labelled. " First experiment to see the 

 " change in position of particles in clay, whose dimensions were 



'changed by pressure. End of 1850 and beginning of 1851." 

 Another is (l A cube of pipe clay and iron scales mixed evenly." 

 The others are flatter and broader pieces, probably representing 

 the same volume with the scales in the pipe-clo/y parallel to the 

 broad surface. There is also a pill box with flaky graphite. 



212. Specimens illustrating the metamorphic origin 

 of mica schist, and the difference between stratification 

 foliation, and cleavage foliation. 



1876. Lent by Dr. H. C. Sorby, F.R.S. 



These are further illustrations of another portion of Dr. Sorby's 

 address. His description of these specimens is : 



" Ripple Drift in slate rocks in which the cleavage cuts the 

 stratification at a considerable angle." 



" Ripple Drift in contorted and highly metamorphosed mica 

 schist, thus proving the original stratified nature of the rock." 



" Mica schist with foliation in the plane of cleavage, developed 

 by compression before the rock was metamorphosed." 



