July 7, 1892] 



NA TURE 



225 



depression in this case, too, in course of time, gives way 

 to a contrary movement. 



It is obvious that elevation and subsidence, in volcanic 

 as well as in sedimentary districts, must alternate, as we 

 indeed observe. Compression, metamorphism, and load- 

 ing cause a negative movement in the sedimentary dis- 

 tricts (geosyncline) ; warming causes elevation ; erosion 

 again creates subsidence. These positive and negative 

 factors at different times have different values, and partly 

 compensate each other. Therefore elevation and subsi- 

 dence are often observed to alternate. 



The greatest contrasts must occur where a highland 

 joins the sea ; here sedimentation and erosion cause a 

 considerable shifting of material ; loading and unloading, 

 as well as great thermal contrasts, dominate in these 

 regions. 



whole complex begins to glide towards the lowland. The 

 sediments get folded to a considerable depth ; faults 

 occurred between districts of diverse motion. The gliding 

 deformation occurred rapidly whenever the base was 

 shaken slightly (earthquakes). The experiment being 

 finished, we let the masses consolidate ; afterwards we may 

 prepare profile-cuts, which may be executed with the saw, 

 if we evaporated plaster. 



The cuts are instructive, if the strata are differently 

 coloured. 



If we mark certain points in the originally level strata, 

 or if we divide the whole system into cubes, we may 

 study the locomotion and deformation of every point, 

 line, square, or cube of our system ; the vertical, as well 

 as the horizontal component of displacement, may be 

 observed and measured. 



Fig. 



The positive and negative movements of the sea-level 

 are not important ; but the amplitude of deformation at 

 the boundary between high land and sea is in some cases 

 as great as 20,000 metres. 



The hydrosphere is relatively constant, whereas the 

 crust executes oscillations of long duration and great 

 amplitude. 



If we want to study in an experiment the formation 

 and motion of a lava-stream, it cannot be our wish to 

 observe the motions of an enormous quantity of a body 

 as viscid and as hot as lava through long time ; that 

 would be mere observation, and not experiment. In a real 

 experiment we observe the motion of a small quantity of 

 a less rigid material for some hours or days. 



If we observe in nature folded strata of hard sandstone 

 and of soft shale or clay, we shall be satisfied to imitate the 

 deformation of the latter masses ; and instead of the hard 

 sandstone, we will take substances as unplastic, but so 

 brittle that they yield to the small forces employed in our 

 experiment. 



So we may produce on a small scale, with application 

 of little force and in a short time, the same effect which 

 we observe in nature on a large scale. 



If we succeed in producing e.xperimentally the same 

 phases of deformation, the same mechanical effect as in 

 nature, if we see fold-chains and complicated eruptive 

 massives growing up with their characteristic features, we 

 shall be obliged to attribute to these experiments a high 

 importance for mechanical geology. 



In my experiments I evaporated muddy material (clay, 

 mud) or plaster of Paris, which consohdates slowly in 

 consequence of an admixture of glue. The strata were 

 differently coloured : some thin strata, consisting of 

 plaster-powder, were brittle, and underwent ruptural 

 deformation, whereas the other masses showed plastic 

 deformation. The whole system reposed on a base, 

 which, according to the plasticity of the material, was 

 inclined by 5° to 15°. 



As soon as the inclination attained a certain limit, the 



The following experiments explain some points in th 

 theoretical essay : — 



The plastic sediments are loaded by a mass, and get 

 deformed in the manner illustrated by Fig. i. The black 

 base and the black side-wall at the right hand (fault scarp) 



Fig. 4. 



are rigid ; the plastic strata are pushed up in form of a 

 fold ; the highest white stratum is rigid, and gets torn 

 into clods. 



Figs. 2-4 show successive stages. A delta, deposited 



NO. I 184, VOL. 46] 



