452 PROF. H. D. ROGERS ON THE LAWS OF STRUCTURE 



The other general rule is, that the foliation is parallel or approximately so to the 

 cleavage, wherever these two structures occur in the same mass of rocks. This 

 fact, recorded by Darwin, of the gneissic rocks and clay slates of South America, 

 has been noticed likewise by Mr D. Sharpe, Mr David Forbes, Mr Sorby, and 

 other geologists in Great Britain, and by the author, in many localities in Southern 

 Pennsylvania, and in other districts of the Atlantic Slope. An interesting in- 

 stance of such parallelism of the foliation to the cleavage, tending to show con- 

 vincingly, that both phenomena are the consequences of one species of force, or 

 only different degrees of development of the same molecular or crystallizing 

 agency, is presented in the great synclinal trough of the lower Appalachian lime- 

 stone, north of Philadelphia. On the north side of this trough, the Primal and 

 Auroral rocks dip southward over a wide outcrop at a very regular angle of about 

 45°. On the south side they have been lifted into, and even a little beyond, the 

 perpendicular position, so that the synclinal axis plane of the belt dips at an 

 angle of 65° or 70° to the south. Neither formation shows cleavage structure on 

 the northern side of the valley, the limestone being there of an earthy texture, 

 and in thick massive beds, but on the south or upturned side, this limestone is 

 altered into a mottled blue and white crystalline marble, and is pervaded with 

 cleavage planes, dipping at angles of 70° and 80° southward. Many parts of the 

 rock are like a foliated calcareous gneiss, thin laminae of mica and talc dividing 

 the slate-like plates of the marble. It is especially worthy of notice that the 

 foliation of these mica and talc, composing some of the thin partings between the 

 original beds of the limestone, is itself very generally parallel to the cleavage in 

 the adjoining calcareous rock. Indeed, wherever the cleavage is excessive, the 

 mass becomes, by introduction of fully developed talc and mica between its laminae, 

 a true foliated stratum. An especial interest annexes to cases of this kind, from 

 their showing, that in the two contrasted conditions of the absence and presence of 

 metamorphism in the two opposite outcrops of the same synclinal fold, both effects, 

 cleavage and foliation, have originated at the same time, and from one and the 

 same cause, and are, in truth, but different stages of the same crystalline condi- 

 tion, superinduced in the mass by high temperature, at the period of its elevation. 

 The above general fact of the prevailing parallelism of the foliation to the clea- 

 vage, is but a corollary of the more general relationship already expressed of the 

 parallelism of the resulting planes of crystallization to the waves of heat, which 

 have produced the metamorphism. 



Examination op the Prevailing Theories of Elevation. 



Perhaps the most current notion respecting the force which has displaced and 

 elevated the originally horizontal strata of the globe, is that which represents the 

 granitic and volcanic rocks as forcibly injected in a melted state into fissures, and 



