XI. An experimental investigation into the flow of Marble

Abstract

That rocks under the conditions to which they are subjected in many parts of the earth’s crust become bent and twisted in the most complicated manner is a fact which was recognised by the earliest geologists, and it needs but a glance at any of the accurate sections of contorted regions of the earth’s crust which have been prepared in more recent years to show not only that in many cases even the hardest rocks have been folded, but that there has often been a marked transfer or “flow” of material from one place to another in the folds. While, however, these facts are undisputed, the manner in which this contortion, with its concomitant flowing, has taken place is matter concerning which there lias been much discussion and a wide divergence ot opinion. Some authorities—among whom Heim* whose work, in Alpine geology must command the admiration of all, may be mentioned—have held that while, in the upper portions of the earth’s crust, rocks, when submitted to pressure, will break, giving rise to faults and overthrusts, the same rocks in the deeper portions of the earth’s crust are unable to break up in this way, owing to the great weight of the superincumbent strata. The lines of fracture become smaller and greatly increase in number, the various minerals constituting the rock thus breaking down into grains, which, however, move around and past one another, the adjacent grains always remaining within the sphere of cohesion. The structure becomes cataclastic ; the rock mass, acting as plastic bodies do, and flowing in the direction of least resistance, maintains its coherence while altering its shape. Heim believes that there is a further stage in the process which he thus describes :— “ Wird die umformende Kraft endlich so gross dass sie anstatt an ein, paar tausend Stellen die Festigkeit durch Bruch aufheben zu können, dieselbe in jedem einzelnen Punkte iiberwindet, so wird das Spaltennetz unendlich fein und das Gesteinskorn zur Kleinheit eines Molekules reducirt, d. h. die mechanische Bewegungseinheit ist nicht mehr ein Gesteinsbrocken sondern unendlich klein so dass die Bewegung einecontinuirliche Umformung ohne Bruch wird.’’