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2. On the Laws of Structure of the more disturbed Zones 



of the Earth's Crust. By Professor H. D. Rogers, of the 



United States. 



After adverting to previous publications on the subject by himself 

 and Professor W. B. Rogers, the author of the paper began the 

 enunciation of the laws of structure of disturbed tracts of strata, by 

 stating the general proposition that in all districts where the strata 

 have been displaced fi'om the original positions or levels in which 

 they were deposited, they invariably have the form of one or many 

 waves, even where, from a flatness of the undulations, they seemed 

 to retain their original horizontality. In large areas of undulating 

 strata, where the dips are gentle, the main or primary crust waves 

 are very broad ; but where the dips are steep, the crests of the adja- 

 cent undulations are more closely approximated, and generally the 

 amplitude of the waves is in proportion to their flatness. 



It is another prevailing feature of districts of displaced strata, that 

 the undulations into which they have been lifted are approximately 

 parallel, and exhibit a remarkable resemblance to those great conti- 

 nuous billows, which are called waves of translation. This wave- 

 like structure was first distinctly recognised by the author and his 

 brother in the Appalachian chain of the United States, and has been 

 subsequently shown by them to characterize other mountain systems, 

 such as the Jura, the Alps, and the mountainous districts of Wales 

 and Belgium, and other countries. 



Parallelism. 

 1. Expressing, in systematic form, the genera! relations of the 

 flexures of the earth's crust to each other, the first law is that of 

 the mutual parallelism of the waves. This prevails not only be- 

 tween adjacent individual flexures, but between these and the chief 

 igneous axes of the disturbed zones, including them. The parallel- 

 ism extends to the different groups of waves into which the breadth 

 of the undulated district is divided, and subsists as well between 

 those which are curved in their crest lines as between those which 

 are straight. The persistency of this law of parallelism throughout 

 the Appalachian chain, was fully exemplified in the paper. The 

 geological maps of the United States and of Pennsylvania, soon to 

 be published, make it obvious upon mere inspection. 



