726 DYNAMICAL GEOLOGY. 



extend to the depth of a yard or more, and are two or three inches 

 wide at top. 



2. Baking effect from the heat of igneous ejections. — The adjoining 

 rock, especially if argillaceous, is often cracked into small columns 

 of five or six sides, or more, while at the same time hardened. 



3. Loss of heat. — Contraction from the loss of heat often pro- 

 duces a reticulation of vertical cracks, which are usually too narrow 

 even for the insertion of a knife-blade, unless the rock contain 

 considerable moisture. To this cause is to be attributed the divi- 

 sion of basalt or trap into columnar forms. (The size of the 

 columns is, however, dependent on concentric crystallization within 

 the mass, as explained on p. 98.) 



4. A removal of the support of rocks by undermining or other causes. 



5. Pressure of a column of liquid rock, as in volcanoes. 



6. Expansive force of vapors, especially when suddenly evolved, as in 

 volcanic regions. 



The preceding are local causes of fracture. The last-mentioned 

 is an exception to this, according to those geologists who attribute 

 to vapors the elevation of mountains. 



7. Tension within the earth's crust, — the same agency which has 

 been explained on a preceding page as the true source of its plica- 

 tions and of the uplifting of mountains. Fractures have been 

 made, through this means, of all extents, from those intersecting 

 single layers, to profound breaks reaching down to regions of inter- 

 nal fires. In the plication of the rocks, fractures are most likely 

 to be produced along the axes of the folds where the flexure is 

 greatest, — those of the upward or anticlinal flexure opening up- 

 ward, and those of the downward or synclinal flexure opening 

 downward. If the latter extend through to the surface, they may 

 give exit to melted rock. In periods of metamorphism, the lateral 

 pressure causing the plications appears in general to have so closed 

 up the fractures made, that igneous ejections were rare. It is not 

 certain that any took place during the metamorphism of the 

 Appalachian region ; though subsequently, after the rocks had 

 been stiffened by crystallization, the sinking of the geoclinal val- 

 leys occupied by the Mesozoic sandstone formation gave origin to 

 a great profusion of trap ejections (p. 430). 



Direction of fractures. — Fractures, in a region elevated by any 

 method of pressure, tend to form, as shown by Hopkins, in a direc- 

 tion at right angles to the line of greatest strain or pressure, and 

 also, in many cases, in a second direction transverse to this, making 

 two systems, a primary and a subordinate. 



