387 



but all of tliem can be reconciled to the general fact, and reduced, it 

 is conceived, to the one comprehensive law, — that the planes of 

 foliation, or the laminae formed by the crystalline constituents of the 

 foliated rocks are parallel to the planes or waves of heat which have 

 been transmitted through the strata. Whenever large tracts of the 

 gneissic rocks retain a nearly horizontal undisturbed position, the 

 foliation is almost invariably coincident with the stratification ; and 

 in this case the wave of heat producing the crystalline structure 

 can only have flowed upwards through the crust, invading stra- 

 tum after stratum in parallel horizontal planes. Again, when in- 

 jections of granite have lifted the gneissic strata, the crystalline 

 lamination is generally seen to be parallel to the plane of outflowing 

 temperature. 



The other general rule is, that the foliation is parallel, or approxi- 

 mately 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. An interesting instance of such parallelism of the 

 foliation to the cleavage, in the last-named region, tending to show 

 convincingly that both phenomena are the consequences of one species 

 of force, or but different degrees of development of the same molecu- 

 lar or crystallizing agency, is presented in the great synclinal trough of 

 the lower Appalachian limestone, north of Philadelphia. On the north 

 side of this trough, the primal and auroral rocks, Cambrian or Lower 

 Silurian, dip S., 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 for- 

 mation 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 la- 

 minae of mica and talc dividing the slate-like plates of the marble. 

 What is especially worthy of notice is, that the foliation of the mica 

 and talc, composing some of the thin partings between the original 



