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. Suarre, Mr Davip Forses, Mr Sorsy, 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 laminze of mica and tale 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 tale and mica between its laminee, 
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 OF 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 
