OF THE MORE’ DISTURBED ZONES OF THE EARTH’S CRUST. 449 
Relation of Cleauage to the Mechanical Constitution of the Strata: 
‘4 There is yet another law respecting cleavage; it is the dependence of this 
structure upon the mechanical texture, and possibly upon the chemical compo- 
sition, of the fissured rocks. 
Geologists have for several years recognized the fact, that in formations com- 
posed of alternations of the coarser mechanical rocks, such as silicious grits and 
conglomerates, with fine-grained argillaceous. beds, as slates, shales; or marls, 
the coarse beds are unaffected by cleavage, while the fine-grained ones are often 
pervaded by it. Indeed, one may observe in a given locality almost. a strict 
proportion between the degree of intimate fissuring of the rocks by cleavage planes. 
and the degree of comminution of their particles. 
Connected probably with this interruption in the distribution of the cleavage- 
condition through such heterogeneous groups of strata, I have observed another 
general fact of modification of the cleavage planes, which should not be passed 
unnoticed. here. They tend in the fine grained argillaceous beds, to curve a 
| little from the normal direction into an approach to parallelism with the surfaces 
of bedding of the adjoining coarser mechanical deposits, presenting, in a trans- 
_ verse section, a kind of gentle sigmoid or double fiexure. This is well shown in 
| the cleavage-traversed rocks at the base of the anthracite coal formation of Penn- 
sylvania, especially in the transition or passage beds which connect the Umbral 
red shales of that region, with the base of the coal-sustaining conglomerate, 
and also where these shales alternate with the upper coarser members of the Ves- 
pertine sandstone. The.small section here appended, showing the cleavage in 
. one of these groups of alternation of red shale and sandstone, from a railway 
_ cut near Ashland, in the middle anthracite coal-field, exemplifies well the pheno- 
menon referred to. 
Fig, 4. 
a qi es Ne Sey 
: s Bs ae NM) See ce 
q Beds of Red Shale with ea alternating with beds of Sandstone without Cleavage; Cleavage curving towards 
parallelism with the bedding at its boundaries. Section mear Ashland, Pennsylvania. 
; ‘The tendency, here shown, in the cleavage planes to conform to the planes of 
bedding, where abrupt changes of composition interrupt the continuity of the 
_ fissures, is but another variety of the phenomenon already adverted to, of a de- 
