— 
_ have been pushed over in a more or less northerly direction,’ 
_ 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 
OF THE MORE DISTURBED ZONES OF THE EARTH’S CRUST. 447 
comprehensive and exact expression, when he stated to the British Association in 
1848, 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 Biytx of the Cleavage of tle 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 
’ inverting the car- 
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 Sarre respecting the cleavage of 
* See Ann. Reports on those Surveys, 1837-40, and other Essays. 
VOL. XXI. PART III. 6E 
