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i XXX.—On the Laws of Structure of the more Disturbed Zones of the Earth's Crust. 
By Professor H. D. Rocxrs. 
(Read 21st April 1856.) 
Having, several years ago, in the course of a prolonged investigation of the 
geological structure of the Appalachian chain of the United States, conducted 
partly in co-operation with my brother, Professor W. B. Rocsrs, as a purely 
scientific inquiry, partly by myself, in connection with a Government Survey of 
the State of Pennsylvania, discovered what we deemed important laws, applicable 
generally to all corrugated tracts of strata; and being prepared, by observations 
since made in the United States and in Europe, to extend their application, and 
give them a more general expression, I have thought that I could not select a 
more suitable subject for my first communication to the Royal Society of Edin- 
burgh, than this portion of descriptive and dynamic geology, which has engaged 
much of my attention, theoretically and practically, for these many years. In 
| presenting an outline of the views already arrived at, and published by us as a 
necessary part of the further generalizations since reached, I will refrain from re- 
| __ peating, in historical detail, what we have already written, but will give our con- 
clusions in the form and with the brevity most compatible with clearness, refer- 
ring to the printed papers and communications where the special topics included 
in this more general summary may be seen. 
Wave-like form of all Upraised Tracts of the Crust. 
The first or most general fact which I would enunciate respecting any portion 
of the earth’s crust that has suffered elevation or depression from the position or 
level in which its strata were originally deposited, is, that the displaced beds 
present invariably the form of one or many wayes, even when within limited 
geographical areas they may seem to retain an approximate horizontality. This 
_ comprehensive statement respecting the wave-like structure of the earth’s crust, 
is not invalidated by the instances of disordered dip seen in certain dislocated 
_ regions, such as some of the coal-fields of Great Britain; for it will generally be 
| _ found that the breaks or faults in the strata only separate disarranged portions 
| __ of what were originally continuous undulations. 
In all large stratified areas, where the dip is both gentle and persistent 
in its direction throughout considerable spaces, and where this dip is genuine, 
the result, that is to say, of a true displacement of the mass, and not a conse- 
quence of the original obliquity of deposition called false bedding, the crust 
waves will be found to be of an amplitude proportioned to their flatness; but in 
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