OF PEBBLES IN CONGLOMERATES. 4S5 



induced upon rigid bodies under restraints and by forces of various kinds ; and indeed iron 

 in some cases passes from a fibrous to a crystalline condition simply from terrestrial magnet- 

 ism. In metallurgical operations certain poor copper ores are brought to a heat, which must 

 not be great enough to melt them, when the copper, which before existed as copper pyrites and 

 was more or less uniformly disseminated through the mass, is found as a kernel in the 

 interior; while portions of the iron and sulphur contained in the crude ore are driven out- 

 wards. Many of the so-called contact changes, seen in the neighborhood of dykes, were 

 certainly produced while the rock was in what we should call a rigid state. Such facts 

 illustrate the remark of Gay Lussac — " Dans un corps solide les molecules peuvent changer 

 le position, et prendre la forme crystalline;" and also that of Keilhau, that "Solid 

 unorganized bodies are capable of changing their structural and chemical characters, by 

 means of slowly working causes in the solid rock, and of assuming forms which they did not 

 before possess." We must indeed suppose that molecular motion is always going on in 

 rocks, inasmuch as this motion is a form of heat, and as no rock is entirely cold. Pressure 

 causes in some cases a quicker molecular motion, or a greater heat, or a less rigid condition, 

 under which a new physical structure may be induced quicker than when the body is more 

 rigid, or colder, or more at rest molecularly. 



The pebble shown in PI. XIX, fig. 1, would seem to have been rigid, to have been so 

 sharply broken ; we should hardly call a body plastic that became ruptured in such a man- 

 ner. As a general rule, however, no such decided breaking is seen ; and from the nature of 

 the assumed operation we should not suppose that it would be. Mr. Tyndall remarks, 

 regarding; the moulding of ice into curved forms, that in every case the ice in changing its 

 form was crushed and broken ; but that if instead of three moulds three thousand had been 

 used, or better still, if the form of a single mould had been changed by extremely slow 

 degrees, no rude rupture would be apparent ; the ice would behave as a plastic mass. And, 

 again, he observes: "How far this ice is bruised and broken, and how far the 



motion of its parts may approach to that of a truly viscous body under pressure, I do not 

 know. The critical point here is, that the ice changes its form, and preserves its continuity, 

 during its motion, in virtue of external force. It remains continuous whilst it moves, because 

 its particles are kept in juxtaposition by pressure, and when this external prop is removed, 

 the analogy with a viscous body instantly breaks clown." Suppose now that these 

 pebbles, in a perfectly rigid condition, are submitted to one of those enormous pressures, 

 which geology has the right to call upon, for a hundred thousand years ; and suppose that 

 such a pebble should yield to the force, and break, ten times each year ; and after each 

 fracture should be jammed together again in a slightly altered form. If the pebble is 

 obliged to bend in the whole time through the space of an inch, it would yield at each frac- 

 ture the one millionth only of that space. What difference could we see between such a 

 movement and that of a really plastic mass? We believe that such a pebble would change 

 its form both by rupture, where subject to tension, and by molecular yielding, where subject 

 to compression, and, as Mr. Tyndall says of the ice, would preserve its continuity, during the 

 change of form, in virtue of external force. It is plain that no rigid pebble can withstand more 

 than a certain force without yielding ; and it is also plain that simple pressure, if great 

 enough, will compact loose materials into solid rock ; and we hold that in the folding up of 

 vast strata, both this breaking and this reconstructing force are necessarily present. 



Mr. Sorby states, that the fine grained slates containing minute rounded grains of mica, sel- 



MEMOIUS HOST. SOC SAT. HIST. Viil I, Pt 3 123 



