50 



CURVED STRATA. 



[Ch V« 



tion wliich is concealed beneath the sea-level, as also that which is sup- 

 posed to have once existed above the present surface. 



We may still more easily illustrate the effects which a lateral thrust 

 might produce on flexible strata, by placing several pieces of differently 

 colored cloths upon a table, and when they are spread out horizontally, 



cover them with a book. Then apply other books to each end, and force 

 them towards each other. The folding of the cloths will exactly imitate 

 those of the bent strata. (See fig. 65.) 



Whether the analogous flexures in stratified rocks have really been 

 due to similar sideway movements is a question of considerable difficulty. 

 It will appear when the volcanic and gi-anitic rocks are described, that 

 some of them have, when melted, been injected forcibly into fissures, 

 while others, already in a solid state, have been protruded upwards 

 through the incumbent crust of the earth, by which a great displace- 

 ment of flexible strata must have been caused. 



But we also know by the study of regions liable to earthquakes, that 

 there are causes at work in the interior of the earth capable of producing 

 a sinking in of the ground, sometimes very local, but sometimes extend- 

 ing over a wide area. The frequent repetition, or continuance throughout 

 long periods, of such downward movements seems to imply the formation 

 and renewal of cavities at a certain depth below the surface, whether by 

 the removal of matter by volcanoes and hot springs, or by the contrac- 

 tion of argillaceous rocks by heat and pressure, or any other combination 

 of circumstances. Whatever conjectures we may indulge respecting the 

 causes, it is certain that pliable beds may, in consequence of unequal 

 degrees of subsidence, become folded to any amount, and have all the 

 appearance of having been compressed suddenly by a lateral thrust. 



The " Creeps," as they are called in coal-mines, afford an excellent il 

 lustration of this fact. — First, it may be stated generally, that the exca 

 vation of coal at a considerable depth causes the mass of overlying strata 

 to sink down bodily, even when props are left to support the roof of the 

 mine. "In Yorkshire," says Mr. Buddie, "three distinct subsidences 

 were perceptible at the surface, after the clearing out of three seams of 

 coal below, and innumerable vertical cracks were caused in the incum- 

 bent mass of sandstone and shale, which thus settled down."* The ex- 



* Proceedings of Geol. Soc. vol. iil p. 148. 



