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bones belonging to the human ear. He tells us, with the authority of a profound and careful observer, “ that the highest type—the human-passes through every stage of morphological structure seen in the series beneath; it does not stop at those stages; it does not utilise, so to say, the incipient structures ready to be so used, but runs rapidly along its own line, choosing, as it were, and refusing, until at length the perfect brain is obtained. Yet this perfection of parts, this production of a creature, who in his lowest attributes is the paragon of animals,' is not brought about irrelatively to the rest of creation; it is merely an elected consummation of all that is highest and best in morphological structure. Does this exclude Teleology,' or the fitness of every part to other parts and of the rest of the world ? I think not.”
In the mechanical department of microscopic science, Mr. J. F. Stanistreet, of Liverpool, has suddenly taken a high rank. He constructed a micro-ruling machine to while away certain hours of tedious illness, and with this he has made exquisite stars and other patterns both on glass and steel. As objects of beauty, these micro-rulings possess great interest, and they are likewise valuable as illustrating some curious optical facts and illusions. As Mr. Stanistreet's machine is figured in the “ Monthly Microscopical Journal,” and as a paper with illustrations will be found therein, describing his exquisite work, our readers may be referred to that source for further information. Mr. Stanistreet was quite unaware of the remarkable mechanical skill he possesses while other pursuits occupied his time. He still thinks his performances easy, though few could imitate them. Perhaps some other folks may be so fortunate as to discover that they possess special aptitudes which have hitherto been allowed to sleep. Specimens of Mr. Stanistreet's work have been presented by him to the Royal Microscopical Society, where they may be seen, on the introduction of a Fellow.
THE geologist is continually engaged in attempting to repro
1 duce a past world. From footprints he tries to reconstruct extinct animals; from accumulated sediments he infers the course of ancient rivers. He seeks to interpret fissures and veins, cliffs and valleys, by referring them to natural agents whose mode of operation is comprehensible. In general, from effects he reasons to causes.
No doubt this process is liable to continual error. The candid geologist will perhaps admit that, biologists excepted, no scientific inquirers have blundered so often and have so often been forced to relay the foundations of their labours as the students of physical geology. The extreme complexity of the phenomena concerned explains the natural tendency of geologists to guess instead of demonstrating.
The only remedy for this source of error is continual verification. When we have assigned a cause to any group of effects we must endeavour to show by tangible evidence that the cause invoked is actually existent, not wholly conjectural, and that it is adequate to the work hypothetically attributed to it.
It needs hardly be said that this process of verification is usually tiresome, and sometimes impossible. We cannot always imitate, even upon the smallest scale, operations which have played a great part in the history of the earth. Even where verification is easy we are too prone to neglect it, and trust to assumption or the application of formulæ. Our present subject of Contortion of Rocks is but one out of many branches of physical geology which has been overlaid with speculation, while the recorded experiments are few.
In this paper I propose to give results of experiments undertaken in the hope of verifying some generally received principles of geology. It will appear, I think, that we can imitate successfully—though on the humblest scale—those great bends and folds of strata which the geologist studies with admiration at Holyhead or Torquay or Fast Castle.
A thick solid bed of limestone bent to a right angle without fracture-how can that phenomenon have been produced ? Such was the question which forced itself upon my notice some years ago when collecting facts respecting a series of anticlinal elevations in Craven. In a limestone quarry at Draughton, between Skipton and Bolton Abbey, I came upon beds of rock a foot or two in thickness bent into the figure of an inverted W. The angles were sharp and unbroken. You might pass your finger over the apex of one to make sure that there was neither crack nor vein. To what force must we assign this disturbance without fracture of strata once hard and horizontally laid ?
Forty years ago it would have seemed natural to invoke a volcanic eruption acting upon plastic matter—to regard such folds and contortions as due to the formation or elevation of a kind of blister upon the earth's surface. But thanks to a few experimental inquirers, such as Sorby, Hodgkinson, and Tyndall, a number of plausible suppositions had been swept away as fallacies. In 1867 the geologist looked rather to lateral pressure (due possibly to contraction of the figure of the earth) as most probably the force concerned, and he did not find it necessary to suppose that the distorted rocks had ever been plastic. It is true that Sir James Hall assumed that the rocks of Berwickshire were ductile when contorted, and Dr. Edward Hitchcock, a well-known American geologist, had recently maintained that some contorted pebbles in a conglomerate at Newport, Rhode Island, must have been as plastic as moist elay when they were bent and twisted. But this gratuitous assumption was soon disposed of. There were a few fossils in the Draughton limestone, and these were distorted like the rest of the rock.
This seemed to prove that plasticity was not a necessary condition of contortion. The shells and corals had surely not been plastic. Indeed the matrix itself may well have been compact rock from the time of its deposition, growing by the addition of hard lumps and shells and films of stony calcareous matter.
A rigid body compressed without fracture into the figure of W—was this possible? There is, as I afterwards discovered, a source of error in the word “rigid ”—a latent hypothesis which turns out to be erroneous. • Rigid " is purely a relative term. Stone is rigid in comparison with clay, but plastic in comparison with cast steel. Absolute rigidity is an unknown property of matter. Let us select a few examples of what are commonly regarded as rigid bodies. Rock crystal, glass, calc spar, steel and limestone, are surely fair specimens. Yet, Tyndall gives instances of quartz crystals altered in shape by pressure, some of them having yielded along transverse planes as if one-half had slidden over the other, but subsequently strongly cemented together by mere apposition and pressure. He regards the action of strongly compressed glass upon polarized light as proof of an alteration in its molecular arrangement. Mr. Sorby has cited examples of distorted crystals of calc spar in cleaved limestones. M. Tresca, in his paper on the “ Flow of Solids," read before the Institution of Mechanical Engineers at Paris in 1867, gives the result of experiments made upon lead, iron, and even steel, and shows that these metals behave like liquids when subjected to adequate pressures. As to limestones and other rocks, I can say from my own experiments that they are both elastic and plastic, yielding more or less to forces of short duration, but recovering their original figure, while when subjected to long-continued pressures or strains of low intensity they are capable of setting permanently in a new shape.
It is curious to observe how speculation has been misled by the notion of absolutely rigid bodies, by the assumption that hard rock can exhibit neither an appreciable elasticity nor any ductile properties. Sir James Hall, of Dunglass, whom Professor Geikie has lately styled “ the founder of Experimental Geology, since it was he who first brought geological speculation to the test of actual physical experiment," investigated the subject of contortion with much care. He had previously carried out laborious inquiries into the influence of pressure in modifiying the action of heat. The curved strata of the Berwickshire coast had engaged his attention since the year 1788. In 1814 appeared his remarks on “ The Convolutions of Strata and their meeting with Granite."* In this interesting paper he describes the local phenomena with some minuteness, and then gives the “ rude experiment” contrived to imitate the conditions which he supposed to have obtained in nature :
“ Several pieces of cloth, some linen, some woollen, were spread upon a table, one above the other, each piece representing a single stratum ; a door (which happened to be off the hinges) was then laid above the mass, and being loaded with weights, confined it under a considerable pressure. Two boards being next applied vertically to the two ends of the stratified mass, were forced towards each other by repeated blows of a
• " Transactions Royal Society, Edinburgh,” vol. vii. pt. 1.
mallet applied horizontally. The consequence was, that the extremities were brought nearer to each other, the heavy door was gradually raised, and the strata were constrained to assume folds, bent up and down, which very much resembled the convoluted beds of killas, as exhibited in the crags of Fast Castle, and illustrated the theory of their formation.
6 I now exhibit to the Society a machine by which a set of pliable beds of clay are pressed together, so as to produce the same general effect; and I trust that the forms thus obtained will be found, by gentlemen accustomed to see such rocks, to bear a tolerable resemblance to those of nature.”
The positions which we may now consider to have been established by Sir James Hall's experiments and reflections are these : That strata originally horizontal have been curved and folded ; and that the disturbing force has acted in a horizontal direction. His further decision that the force concerned is necessarily volcanic may be questioned. The absence of superficial traces of volcanic agency over large areas of contorted strata—the limestone district of Craven, for example-–is not easily reconciled with the views derived by Hall from his instructor, Hutton. We must also emphatically dissent from his tacit assumption that the contorted rocks must have been “in a soft but tough and ductile state.” Distorted fossils, crystals, and pebbles cannot well have been soft when they were pinched and bent out of shape. Nor need we assume such a condition during the formation of ordinary curved strata. The mechanical properties of limestones and other rocks, dry and at ordinary temperatures, are such as in themselves satisfy the conditions of the problem.
It is natural that early experimenters should fail to perceive many important aspects of the questions which they propose to themselves. A highly interesting addition to Sir James Hail's researches on the influence of pressure as modifying the action of heat was made by Faraday, who showed that the pressure of fifty atmospheres, believed by Hall to be requisite to prevent the escape of carbonic acid from limestone during the process of crystallization by heat, is not indispensable. The composition of the surrounding gas affects the facility of dissipation, and in an atmosphere of carbonic acid, fragments of limestore may be crystallized by heat at standard pressure. The student of geology will not need to be reminded of the importance of this qualifying consideration in connection with calcareous deposits in recent volcanic rocks. Similarly, in his contortion experiments, Hall stopped short at an early stage. It does not appear to have occurred to him that pieces of the very rocks whose curvature he was investigating might be made to bend permanently by means of the same apparatus employed in