446 



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



German side. Italian side- 



Generalized Section of the Alps, displaying the dipping of the folds of the strata on both sides in towards the 



igneous axis. 



a, Igneous rocks in central axis of the chain. b, Gneissic and other older strata. 



c, Anticlinal flexures. d, Synclinal flexures, or Vs. e, Anticlinal and synclinal axis planes. 



A conspicuous and pervading cleavage structure coincident in the direction of 

 its dip, as I shall presently show, with the oblique axis planes of the folded 

 rocks, contributes greatly, I conceive, to the illusive phenomenon of an inward 

 dip of all the strata, or to that general feature which has been called fan- 

 shaped.* 



This inward dip is rendered still more obvious by the circumstance, that the 

 foliation or crystalline lamination of the more altered strata, itself obeys very 

 generally a similar law of parallelism to the axis planes of the flexures. Where 

 this crystalline grain of the rocks does not coincide with the stratification, it 

 exhibits a great tendency to a coincidence of dip with any system of cleavage 

 planes belonging to the same or other parts of the mass. In either case, it will 

 dip inwards towards the igneous axis of the chain, if the strata possessing it are 

 themselves closely folded in conformity with the prevailing law. But the phe- 

 nomena of cleavage and foliation will be noticed afterwards. We now proceed to 

 discuss 



General Phenomena of Slaty Cleavage in the Appalachians and other Zones of 



Plicated Strata. 



Cleavage parallel with, but independent of the Dip of the Strata. 

 It is now a good many years since Professor Sedgwick and other geologists an- 

 nounced the important general fact, that the structure called cleavage pervades the 

 altered strata affected by it, in directions independent of their bedding or laminse 

 of deposition. That eminent geologist further announced that these planes are 

 approximately parallel to each other over large spaces of country, however con- 

 torted the dip of the rocks. He likewise enunciated a second general law of much 

 importance, " That when the cleavage is well developed in a thick mass of slate 

 rock, the strike of the cleavage is nearly coincident with the strike of the beds." 

 Subsequently Professor Philips gave to this rule of the cleavage a still more 



* From the analysis above given of the structure of the sides of the Alps, it will be seen, that 

 I entirely concur with Professor James Forbes, and with all the more eminent of the Swiss geo- 

 logists, in recognizing the fan-like dip of the newer strata, Tertiary and Mesozoic, conformably in 

 appearance at least under the older strata, metamorphic and gneissic, of the higher more central 

 tracts, and that I dissent entirely from the theoretical section offered by Mr Dantfl Sharpe- 



