484 G - L - VOSE ON THE DISTORTION 



horizontal compression, similar to that which has produced the more or less closely folded 

 waves of the Appalachian Mountains. 



The second position of the Messrs. Hitchcock — that the materials at the time of their 

 compression were in a plastic condition — has been strongly objected to, and we think upon 

 good grounds. The statement as made by them, however, has undergone a great modifica- 

 tion since 1860. At that time a plasticity as great as that of moistened clay was suggested. 

 At the meeting of the American Association at Burlington, in 1867, Mr. Charles Hitchcock 

 proposed a plasticity perhaps no more than is implied by a thorough, warm, aqueous inter- 

 penetration of the masses. Just what sort of a physical condition this last-named plasticity 

 may denote, we do not know ; but inasmuch as it is a less plastic condition than that of 

 moistened clay, we believe it to be a more likely one, and we believe it to be more likely 

 still that the materials have not been plastic at all ; but that they were altered both in form 

 and mineral character, while in what may be called a rigid condition. It is well known in 

 the arts that complete changes, both of external form and of internal physical structure, may 

 be produced while bodies are what may be properly called rigid. Mr. Tyndall, in his work 

 upon the Glaciers, refers to certain quartz stones in the Museum of the Government School 

 of Mines, in London, which have been subjected to enormous pressure, and which have become 

 mutually flattened and indented : some of them, he observes, have yielded along planes passing 

 through them, as if one half had slidden over the other, but being strongly reattached ; others, 

 which have endured pressure at a particular point, are fissured radially around this point; a 

 case which may be considered illustrated in PL XIX, fig. 1 ; and he remarks, that the whole 

 collection is an instructive example of the manner and extent to which one of the most rigid 

 substances in nature can yield on the application of a sufficient force. Mr. Tyndall further 

 states, that the molecular arrangement of glass, pressed by a sufficient weight, is actually 

 changed, — the fact being shown by the action of such squeezed glass upon polarized light ; 

 and he considers that a prism of glass might be compressed under a counter and lateral 

 restraint great enough to keep it from flying to pieces, so that it should be flattened, partlj 

 through rupture, and partly though molecular yielding, so as to change its form, and yet 

 present a firmly coherent mass when removed from its confinement. Now we believe this to 

 be the condition under which the conglomeratic pebbles have been placed, namely : immense 

 but extremely gradual compression, and at the same time a counter-pressure sufficient to force 

 the broken mass together in a new form. The crushing and regealing of ice is an operation 

 somewhat of the same kind, and though we may not have in rock the internal liquefaction, 

 which Mr. Tyndall's experiments showed in ice, we do have an internal heat developed in 

 the rock by pressure. Mr. Sorby states that his microscopic investigations proved that 

 mechanical force had been in certain rocks resolved into chemical action. What more can 

 be needed to produce the distortion seen in the pebbles, than a force great enough to break 

 them, and a counter-force sufficient to bring the parts near enough for adhesion in a new 

 position, to which we may add heat and its correlated chemical forces ? It is a well-known 

 fact, that the old-fashioned monumental tablets seen in burying-grounds, consisting of a 

 horizontal slab, generally of marble, supported only at the four corners, and thus subject in 

 the central part to the action of its own weight, become after a few years deflected to a 

 considerable extent. Here we have a rigid piece of rock actually bent, under a force and 

 in a time which, geologically considered, are utterly insignificant. Engineering science is 

 full of examples where permanent changes of form, of size, and of internal structure, are 



