October 2, 1899.] 



KNOWLEDGE. 



235 



SECRETS OF THE EARTH'S CRUST. 



By Gbenttlle A. J. Cole, m.r.i.a., f.g.s., Professor of 

 Geology in the Royal College of Science for Ireland. 



v.— THE GREAT EARTH-MILL. 



THE movements of the solid ground have always 

 excited considerable interest. The terrifying 

 sensations imparted by ordinary earthquakes, the 

 visible rents that in some cases remain after the 

 shock has passed, the slipping of huge rock-masses 

 which were already in a state of strain, conspire to impress 

 whole populations with the instability of the crust beneath 

 them. The earlier geologists were only too willing to magnify 

 these effects, and to explain by tremors and shatterings all 

 the prominent features of the globe. When strata of 

 identical character appeared on opposite sides of a great 

 vaUey, it was asserted that they had become separated by the 

 abrupt opening of a fissure. The vast taluses on the slopes 

 of mountains were said to be due to similar cataclysmic 

 action ; and, indeed, the rock-falls of the liossberg and the 

 Dobratsch gave a good deal of colour to the statement. 

 The ocean basins were thought to be formed by the swift 

 destruction of old continents, while the earthquake-rollers 

 that occasionally flood the Pacific coast were regarded as 

 a picture in little of the agency that brought our marine 

 fossils in past times high upon the land. 



The fact that the crust behaves at present in a far 

 milder manner did not trouble these heroic authors. In 

 those days, each successive fauna in the stratified series 

 seemed to represent a new creation ; the life of the globe 

 had been every now and then swept away, to be remoulded 

 and reproduced upon a nobler scale. Man happened to be 

 living in a period of rest, specially provided for his needs. 

 The next catastrophic movement would be, in all probability, 

 his day of judgment and of doom. 



The extreme uniformitarianism introduced by Hutton 

 still allowed of recurrent catastrophes, whereby the worn- 

 down land surfaces were somewhat swiftly elevated, and 

 were brought up again to cope with the agents of denudation. 

 Lyell,* however, treated the matter from the observational 

 point of view, and concluded that the greater movements 

 of the crust might have resulted from the cumulative effect 

 of small ones. These small movements, he urged, were 

 spread over long periods of time. 



Since the publication of his " Principles of Geology," 

 observations in every quarter of the globe have negatived 

 the suggestion of world-wide, or even continental, catas- 

 trophes in the past. The recognised succession of living 

 things is in itself incompatible with the violent rending of 

 the crust. At the same time, certain epochs may be 

 indicated in which the formation of mountains, and 

 therefore, in all probability, of oceanic hollows, went on 

 more rapidly than at others. V.'hile it would be rash to 

 assume that the surface is now in any degree stable, it 

 may be true that we enjoy a period of calm, following on 

 the amazing earth-storm of later Miocene times. 



Marked changes of level have occurred since man 

 appeared upon the earth' ; we may infer, then, that the 

 crust is not truly at rest at the present time. The earth- 

 quakes from which certain regions suffer are interruptions 

 in the general movement ; and the volcanoes which are 

 often associated with such areas are parallel manifesta- 

 tions of the earns movement. The earth-pressure may 



.• "Principles of Geologv," Vol. I. (1830), p. 79. 



t See "The Edge of a Continent,' Kxowledge, Tol. XX. (1897), 

 p. 208. 



even be in many cases responsible for the outflow of the 

 lava. 



Miners and geologists are familiar with "faults," or 

 planes of fracture In the crust. On one side of the plane 

 the rocks have been lowered : on the other they have been 

 moved upward. The vertical amount of movement, the 

 " throw " of the fault, may vary from an inch up to several 

 thousand feet : and it varies in different parts of the same 

 fault-plane, until at last the fracture dies away. If we 

 take a sheet of paper and cut a slit in it with a knife, 

 leaving the edges of the sheet intact, we can bend down 

 the paper on one side of the slit to a considerable extent, 

 without seriously deforming the whole sheet. Such a slit 

 may illustrate how faults, which arise in the rock-sheet 

 known as the earth's crust, may have a marked throw at 

 one point and a diminished throw towards either end. 



Such fractures, when viewed in a quarry-section, seem 

 at first to appeal to the catastrophist. Here we find the 

 rocks actually dragged asunder. A crushed •'breccia," 

 containing fragments of the rocks on either side, has been 

 formed along the fault-plane, and testifies to the amount 

 of force employed. Those who have wittily called faults 

 "fossil earthquakes," have, indeed, ample justification; 

 for the jar caused by the slipping of rocks along a fault- 

 plane is, no doubt, the source of many a serious shock. 

 In several cases, as in the -Japanese earthquake of 1891, a 

 fault is made visible at the surface ; but it is of course 

 possible that a larger and concealed movement may have 

 caused the earthquake, and may have allowed the surface- 

 rooks to sUp at the same time. 



Yet the whole movement along a fault-plane is probably 

 one of slow degrees. Repeated sUps have produced an accu- 

 mulative effect ; and in some cases the final displacement 

 must be regarded as differential, movements in opposite 

 directions having occurred in different parts of the same 

 fault-plane, and therefore possibly in the same part. The 

 faulting is in reality so slow that some of the larger 

 fractures are known to belong to more than one geological 

 period. In the French Alps, M. Lory* has remarked the 

 continuous deposition of sediments on the sinking side of 

 a fault-plane, the movement continuing from the Oxfordian 

 to the Senonian epoch. Similar movement probably 

 accounts for the great thickness of the Old Red Sandstone, 

 deposited, layer by layer, in central Scotland, in the floor 

 of a lake bounded by fault-planes ; and the modern history of 

 Lake Tanganyika points also to continuous faulting along its 

 margins. These slow movements form part of the work of 

 what we may call " the great earth-miU," whereby the 

 surface-rocks are shifted on one another, while those lower 

 down may actually be ground to powder. 



The breccia found along a plane of faulting is known as 

 the " fault-rock. ■ The fragments in it art -jrooved by 

 friction with one another : the more yieldmg ones are 

 squeezed out of shape, or are dragged out Into mere wisps 

 and films. In some places, a rough flow of the fractured 

 solid may be observed, the more finely powdered material 

 having moved round the larger blocks, which have thus 

 become embedded in a groundwork, in the form of knots or 

 " eyes." 



The same features may be observed in certain yielding 

 rocks, such as serpentines or limestones, on a broad and 

 considerable scale. Earth-movements may cause these 

 masses to be torn to pieces, and the faulting or "shearing" 

 at any point is obscured by the quantity of fault-rock 

 produced. The magnificent brecciated marbles of Italj, 



* See the admirable discussion b_v Dr. Davison, in " The Hereford 

 Earthquake of 189G" (Birmingham, 1&99), p. 2C5. 



t -See De Lapparent " Traite de Qeologie," 2me. ed., p. 1406. 



