OF THE MORE DISTURBED ZONES OF THE EARTH'S CRUST. 447 



comprehensive and exact expression, when he stated to the British Association in 

 1843, that the cleavage planes of the slate rocks of North Wales were always pa- 

 rallel to the main direction of the great anticlinal axes. Other geologists have 

 abundantly confirmed these generalizations. Since 1837, these phenomena of the 

 close parallelism of the cleavage planes of a given district with each other, and 

 with the main axis of elevation of the district, have been constantly observed and 

 recorded by my brother Professor W. B. Rogers and myself, in our Geological 

 Surveys of Virginia, Pennsylvania, and New Jersey.* 



In 1849, I submitted to the American Association for the Advancement of 

 Science, at the annual meeting held at Cambridge, Massachusetts, in a commu- 

 nication on the analogy of the ribbon structure of glaciers to the slaty cleavage 

 of rocks, a statement of what I had for some years past regarded as the true 

 law of the direction and position of the cleavage planes of a district of undulated 

 and plicated strata. 



In its simplest expression the rule is, that the cleavage dip is parallel to the 

 average dip of the anticlinal and synclinal axis planes, or those bisecting the 

 flexures. The generality of this rule was shown on the occasion mentioned, by 

 sections exhibiting the flexures and cleavage in the Appalachians, in the Alps, 

 and in the Rhenish Provinces ; and I have since become convinced of its univer- 

 sality from the inspection of the phenomena of other districts, and from a study 

 of the descriptions and sections of geologists. Want of time at present prohibits 

 me from citing the abundant evidence for this law to be found in the best recently 

 printed memoirs upon slaty cleavage ; but I hope to be able ere long to give my 

 own observations in support of the highest British geological authorities, who, 

 unaware of the relationship itself, have furnished the most satisfactory data for 

 the recognition of it. I cannot, however, refrain, in this place, from sustaining 

 the generalization I am here venturing to put forth, by instancing the support it 

 receives from the excellent descriptions recently given by Professors Harkness 

 and Blyth of the Cleavage of the Devonians of the South-west of Ireland. 

 In their paper in the Edinburgh New Philosophical Journal for October 1855, 

 they not only establish an agreement between the strike of the cleavage planes 

 with that of the several rolls (or anticlinals) which affect the island of Valentia, 

 but they show that while the cleavage dip is southerly, the anticlinal " curves 

 have been pushed over in a more or less northerly direction," inverting the car- 

 boniferous limestones and coal measures. Their general statement is, that the 

 cleavage structure of rocks does not result from the simple rolling of the strata, 

 but from this cause combined with a considerable amount of pressure ; and this 

 latter force acting from the south, has pressed over the strata in a series of oblique 

 curves to the north, and given to the inclined cleavage its more or less of a south- 

 ern dip. They support the doctrine of Mr Sharpe respecting the cleavage of 



* See Ann. Reports on those Surveys, 1837-40, and other Essays. 

 VOL. XXI. PART III. 6 E 



