388 



beds of the limestone, is itself very generally parallel to the cleav- 

 age in the adjoining calcareous rock. Indeed, wherever the cleav- 

 age is excessive, the mass throughout becomes, by introduction of 

 fully developed talc and mica between its laminae, a true foliated 

 stratum. An especial interest attaches itself to cases of this kind, 

 from their showing, in the two contrasted conditions of the absence 

 and presence of metamorphism in the two opposite outcrops of the 

 self-same synclinal stratum, that 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 condition, 

 superinduced on tlie mass by high temperature, at the period of its 

 elevation. 



The above enunciated geneial facts of the prevailing parallelism of 

 the foliation to the cleavage, is but a corollary of the still more ge- 

 neral relationship already expressed of the parallelism of the resul- 

 tant planes of crystallization to the waves of heat, which have 

 produced the metamorphism. 



Theoretical Views. 



Theory of the Flexion and Elcv ition of Undulated Strata. 



The wave-like structure of tlie Appalachian and other undulated 

 zones has been attributed by the author and his brother W. B. 

 Rogers, in their communications to the American Association in 

 1842, and to the British Association in the same year, to an actual 

 undulation of the supposed flexible crust of the earth exerted in 

 parallel lines, and propagated in the manner of a horizontal pulsation, 

 from the liquid interior of the globe. They have supposed the strata of 

 such a region " to have been subjected to excessive upward tension, 

 arising from the expansion of molten matter and gaseous vapour, and 

 this tension relieved by parallel fissures formed in successive lines 

 through which such elastic vapour escaped, the sudden removal of the 

 pressure adjacent to the lines of fracture, producing violent pulsations 

 on the surface of the liquid below. This oscillating movement would 

 communicate a series of temporary flexures to the overlying crust, and 

 these flexures would be rendered permanent (or keyed into the forms 

 they present) by the intrusion of molten matter. If, durinor this oscil- 

 lation, wo conceive the whole heaving tract to have been shoved (or 



