OF THE MORE DISTURBED ZONES OF THE EARTH S CRUST. 461 



actual rotation of its parts, such as the mechanical theory assumes ; and I cannot 

 see why one uniform condition of aggregation should not be the result. 



2. In the second place, it assigns no reason for the presence of cleavage planes 

 in fine-grained argillaceous and calcareous rocks, and their absence in silicious 

 ones, both fine-grained and coarse, even when the two classes alternate with each 

 other in intimate parallel contact, where they must have been exposed to precisely 

 the same pressure, both in direction and in amount. In other words, there is no 

 relation discoverable between the known susceptibility of different materials to 

 cleavage, and their susceptibility to compression. But on the other hand, some 

 of the most compressible are the least subject to this peculiar structure. The 

 different susceptibilities of different kinds of mineral matter to molecular polarity 

 is, I conceive, the true explanation of this marked contrast in rocks. 



3. Another quite conclusive objection, I conceive, to the pressure theory of 

 cleavage is, that it fails to show how the cleavage-traversed strata can have re- 

 ceived the pressure in one constant direction, and under an equalized intensity, 

 through all the contortions and bendings which we know they must have pos- 

 sessed before cleavage was imparted to them. It is obvious that no mechanical 

 pressure, come from what quarter it might, could transmit itself uniformly 

 through convex and concave curves, through bodies of rock placed edgewise and 

 flatwise towards it ; but, on the contrary, dynamic considerations must convince 

 us that the resultants of such a pressure would be as various in their directions 

 within the mass, as the ever-changing planes of the corrugated stratification. Not 

 only would the posture of the strata at any point next the quarter of the primary 

 pressure influence the form and direction of the resultant planes of pressure 

 at that point, but the differences in pliability of the different layers compressed 

 would greatly modify them. In other words, while the dip of the cleavage planes 

 within even wide limits, is usually remarkably constant, whatever the contor- 

 tions of the strata, any pressure transmitted through these contortions must be 

 as various, in different portions of the flexures, as the innumerable resultants pro- 

 duced by the ever-varying resistances and the pressure combined. 



4. A like difficulty opposes itself to the pressure theory, in the constancy of 

 the direction of the elongation or stretching of the mass in the line of its 

 cleavage dip. This extension, well expressed by Professor Phillips as a " creep- 

 ing movement of the particles," seen not only in the fibrous grain of cleavage 

 slates, but in the distortion of imbedded fossils, and of the whole substance 

 of the rock indeed, — ascribed to mere compression by the authors above cited, 

 but attributable, I think, to an actual molecular movement of the mass, in 

 obedience to crystallizing polar forces, — is so equally graduated in amount, and 

 so wonderfully constant in direction (never deviating much from the line of dip 

 of the cleavage plane), that it could never have acquired this constancy from a 

 merely lateral mechanical force, liable to infinite modification, in both of these 



