ANNIVERSARY ADDRESS OF THE PRESIDENT. IxXV 



siderable compression of the mass of rock between the planes of 

 cleavage ; that is, in the direction corresponding to that of the eleva- 

 ting force : this compression being shown by the distortion of the in- 

 cluded organic remains and the flattening of the component portions 

 of the rock, and bearing a proportion to the degree in which the 

 cleavage is developed. The compression of the mass in a direction 

 perpendicular to the cleavage has been partially compensated by its 

 expansion along the dip of the cleavage, in which direction only its 

 expansion was permitted as the elevation of the beds enlarged the area 

 occupied by them. The difference between the amount of compression 

 in one direction and expansion in another, is accounted for in the 

 greater density of the rock after compression. No connexion has 

 been detected between cleavage and crystallization beyond a tendency 

 of plates of talc and mica to arrange themselves along the planes of 

 cleavage ; but as on these planes there would be the least resistance 

 to their intrusion or formation, this may have been a subsequent ope- 

 ration, and should not alter our opinion of the cause of the cleavage." 



You will necessarily refer to the communication itself for the facts 

 considered to justify these views, and take them in connexion with 

 the former memoir on the same subject by Mr. Sharpe. The frag- 

 ments found in certain brecciated beds of Patterdale, Langdale, and 

 other places, are inferred to have been flattened by the pressure con- 

 sequent on the elevation of the strata, and to have been thus squeezed 

 into planes parallel to those of the cleavage, therefore by the same 

 force which lengthened organic remains in the cleavage planes of cer- 

 tain fossiliferous rocks. Though difficult otherwise to observe, it is 

 stated that by means of a lens the constituent particles of good roof- 

 ing slates are found to be " flattest between the cleavage planes and 

 longest along the dip of the cleavage." The double cleavage, pro- 

 ducing the prismatic solids worked for slate-pencils near Shap, is 

 inferred to have resulted from a second compression. 



In the address for last year attention was called to a memoir by 

 Mr. Hopkins on the internal pressure to which rock masses may 

 have been subjected, and its possible influence on the production of 

 laminated structure, a memoir to which we would again request your 

 attention with respect to the arrangement of the component parts of 

 strata consequent on their pressure, bending, and contortion. 



There can be little doubt that it is highly needful, when we regard 

 a mass of rocks, divided by cleavage planes, thoroughly to weigh the 

 effects that may be due to mechanical pressure alone. When there- 

 fore we find beds, so cleaved, to have been removed from horizon- 

 tality, or that approach to it which we infer the original mud, silt, 

 sands, or gravels to have taken from deposit, we have in the first place 

 to inquire what would be the effect of the amount of elevation, bend- 

 ing, or contortion observed, regarding the mass of rocks on the large 

 scale. In so doing we have carefully to examine the rocks them- 

 selves, to see if we can learn, from the structure of the component 

 beds, the probable cohesion of the parts of the various strata when 

 the elevation, bending and contortion were effected. More can often 

 be seen for this purpose than might at first sight appear probable. 



