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XXX.—On the Laws of Structure of the more Disturbed Zones of the Earth's Crust

Published online by Cambridge University Press:  17 January 2013

Extract

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, Rogers, 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 Edinburgh, 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 repeating, in historical detail, what we have already written, but will give our conclusions in the form and with the brevity most compatible with clearness, referring to the printed papers and communications where the special topics included in this more general summary may be seen.

Type
Research Article
Copyright
Copyright © Royal Society of Edinburgh 1857

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References

page 433 note * See Abstract of Communication to American Association for Advancement of Science Cambridge, Mass., March 1849, p. 113.

page 435 note * See Dumont's Memoir sur les Terrains Ardennais et Rhenan, &c.

page 439 note * Transactions of the Association of American Geologists.

page 446 note * 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 geologist, 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 Daniel Sharpe.

page 447 note * See Ann. Reports on those Surveys, 1837–40; and other Essays.

page 451 note * In a communication submitted to the American Association for the Advancement of Science in 1849, I attempted to show this analogy of the ribbon structure of glaciers to the slaty cleavage of rocks, in the following remarks:—“The ice of glaciers consists of thin alternate parallel bands or plates of blue crystal ice, and white porous ice, each not more than one-third or one-half of an inch in thickness. These pervade the whole mass of every glacier, and are clearly exposed on the sides of the transverse fissures. Near the sides of the glacier, they are almost absolutely parallel with its mountain walls, but they sweep away towards its medial line, and form, like all the other planes which divide the glacier, a series of innumerable loop-like curves. This looped or festooned form is obviously caused in part by the downward tendency of the movement or flow of the semiplastic ice, and in part by the influence of the terminal moraine to induce that parallelism to itself, which the rocky sides of the glacier produce in the ice near them. The most general fact noticeable in relation to these structural planes, is the approximate parallelism to the rocky walls and terminal moraine confining the icy mass; or in other words, to the surfaces of higher temperature, which inclose the glaciers. However the direction of the ribbon lines may alter by irregularities in the onward flow of the glacier, their position near the region of the nevé is strictly parallel with the surface of the warmer mountain sides.”

page 453 note * It is in consequence of this natural expansion of surface-cracks outwards in anticlinals, that the miner so frequently finds his mineral lodes contracting or dying out as he descends. Several striking instances of this thinning of veins downwards could be cited, from the mines of the United States, situated in anticlinal flexures.

page 456 note * See Lyell's Elementary Geology, 5th Ed., p. 50.

page 458 note * Geol. Trans., 2d Series, iii., p. 461.

page 458 note † Lyell's Manual, p. 610.

page 458 note ‡ Report British Association, Cork. 1843.

page 458 note § Quarterly Jour. Geol., viii., p. 87.

page 459 note * See Geological Observations on South America.

page 460 note * Lyell., p. 612.

page 460 note † See his paper, Quarterly Journal, Geological Society, 1855.

page 460 note ‡ See his Paper for a good figure of deflection of cleavage and foliation in the margin of a vein of quartz.

page 466 note * There is a similar instance cited by Professor Phillips, I think in his Geology of Yorkshire.