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



a time, fails altogether to show us how this pressure could have shifted to new and 

 parallel lines, or how it could take up new positions, and exhibit that relation 

 of relative distances constantly widening, which is seen in all undulated belts. 

 Besides, how could a simple upward pressure along a line in the crust, form a 

 defined or limited anticlinal flexure ? Whether the pressure were exerted by a 

 liquid or a solid subterranean mass, it would produce rather a wide general 

 moderate elevation, than a narrow, sharp, anticlinal wave. 



If, again, this vague theory be modified to admit the action of a series of 

 linear simultaneous pressures, coincident with the observed anticlinal flex- 

 ures of an undulated district, it is not possible to understand why, being conti- 

 guous, they should not all conspire to lift the outer mass or crust into one 

 general bulge or broad distended dome, rather than into a series of alternately syn- 

 clinal and anticlinal waves. In addition to these difficulties, this notion of self- 

 awakened lines of pressure, contains no clear hypothesis of the origin of the linear 

 forces. 



Hypothesis of Corrugation from Sinking of Tracts of the Earth's Surface. 



Another theory of the cause of flexures in the Crust conceives them to have been 

 produced from a sinking of the ground by removal of matter by volcanoes, or by 

 the contraction of argillaceous rocks by heat and pressure. Sir C. Lyell, who 

 appears to advocate this view, supposes that pliable beds may, in consequence of 

 unequal degrees of subsidence, become folded to any amount, and have all the ap- 

 pearance of having been compressed by a lateral thrust; and the creeps in coal- 

 mines are adduced as affording an excellent illustration of this fact.* With every 

 respect for this eminent geologist's ingenious views, I must confess that this con- 

 ception seems to me quite as much beset with difficulties as the somewhat kin- 

 dred theory of elevation and simple upward protrusion. Apart from the objec- 

 tions that it supplies no cause for the peculiar shape of the crust waves, nor any 

 explanation of their parallelism, and their remarkable laws of gradation, it appears 

 to me quite inadequate to account for lateral corrugation at all, or for more than 

 a very insignificant amount of it. A downward pressure or tension over a single 

 area, produced by release of support arising from vacuities beneath the surface, 

 ought not to engender, on any known mechanical principle, a series of flexures, 

 either within or around the area, but should result in a mere subsidence or flat- 

 tening of the portion from whence the support has been withdrawn. If the 

 centering of a very flat dome, too weak to sustain itself, be removed, the dome 

 either suddenly collapses with a fracture, or it indents itself, and sinks where it 

 is weakest and most yielding, till it meets the supporting floor. Before the wide 

 nearly level dome of a segment of the earth's crust can corrugate either itself 



* See Lyell's Elementary Geology, 5th Ed., p. 50. 



