385 



strike of the cleavage planes and that of the several rolls (or anti- 

 clinals) 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 

 carboniferous limestones and coal measures. Their general state- 

 ment 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 pushed over the strata in a series of oblique curves 

 to the north, and given to the inclined cleavage more or less of its 

 southern dip. They further support the deductions of Mr Sharp, 

 " that there has been a compression in the mass in a direction 

 everywhere perpendicular to the planes of cleavage, and an expan- 

 sion of the mass along these planes in the direction of a line at right 

 angles to the line of incidence of the planes of bedding and clea- 

 vage." But from this view of the mechanical nature and the direction 

 of the force engendering cleavage the author of this communication 

 begs leave to dissent. 



A second general law is, that where the cleavage is fully deve- 

 loped, and the anticlinal and synclinal flexures are also conspicuous 

 and very sharp, the cleavage planes immediately adjoining these 

 bondings are not parallel to the axis planes, but radiate partially 

 from them, in a fan-like arrangement, upward in the anticlinals, and 

 downward in the synclinals. This aberration from the normal direc- 

 tion is, furthermore, not symmetrical upon the opposite sides of the 

 geometric axis planes, but is usually greatest upon the inverted or 

 steep sides of the waves. 



A third prevailing relation of the cleavage planes is — their ten- 

 dency to deviate from the normal direction of parallelism to the axis 

 planes, in order to conform partially to the direction or dip of the 

 strata ; and as in every belt of uniform flexures closely plicated with 

 inversions, the uninverted, or normal dips, greatly exceed the in- 

 verted ones in breadth, there prevails a lower inclination in the 

 planes of cleavage than belongs to the planes bisecting the flexures. 



There is yet another law modifying cleavage, dependent upon the 

 mechanical texture, and possibly the chemical composition, of the 

 strata. In formations composed of alternations of the coarser mecha- 

 nical rocks, such as siliceous grits and conglomerates, with the finer- 



