466 



SCIENCE PROGRESS. 



that a rock prism submitted to lateral pressure undergoes 

 elevation, and that the portion which passes outside the 

 region of compression ceases to be acted upon, and ap- 

 parently expands, giving rise to an overthrust on the sur- 

 face, but at the same time fault planes are produced, along 

 which the overthrusts are formed, having their dip inwards. 

 This, therefore, he considers explains, not only the over- 

 thrust of the older beds over the younger, but also the 

 occurrence of such on both sides of a mountain chain with 

 their dips in opposite directions. Owing, however, to the 

 very variable character of the rock masses, the simplicity of 

 fracture displayed in this experiment is complicated by 

 minor movements tending to hide the facts of develop- 

 ment. 



Fig. 2. — Illustrating the production of overthrusts along fault planes 

 formed during lateral compression, as displayed in Daubree's 

 experiment. 



(3) Although generally overthrust planes have the 

 same strike as the folds, yet in some cases slight diver- 

 gences from absolute parallelism occur. These are known 

 from all regions, and show that though both are originated 

 by the same forces, the folds were produced earlier, and the 

 change of direction in the formation of the overthrust arises 

 from increased compression caused by augmentation of the 

 pressure action. 



(4) The dip of the overthrust planes almost invariably 

 differs from that of the strata or folds, sometimes merely 

 cutting across a trough or saddle at an acute angle to the 

 axis plane ; and at others so obtuse to that plane that they 

 pass direct from an arch into the adjoining trough. This 

 proves, therefore, that the effort which produced them is 



