xii The Crust of the Earth 219 



planes of slate, by means of which thin slabs can be split 

 off, are sometimes at right angles to the planes of bedding. 

 In more rigid rocks the strain during upheaval is relieved by 

 the strata cracking more or less nearly at right angles to the 

 planes of bedding. When these cracks, which are origin- 

 ally extremely narrow, sometimes invisible, simply traverse 

 the rock without any distortion (fine lines JJ) they are 

 termed joint-planes, and it is on account of the existence of 

 joint-planes in all rocks that the quarrying of stones is 

 possible without continual blasting. Igneous rocks show 

 joints, probably the result of contraction in cooling after 

 solidifying. The fine hexagonal columns of basalt cliffs are 

 outlined by joint-planes produced by the uniform cooling of 

 a great mass of rock, the interior of which is brought into 

 a state of tremendous tension by contraction until relieved 

 by cracking into columns. A layer of wheat starch on 

 drying is strained in exactly the same way by contraction 

 throughout the mass, and similarly cracks into many-sided 

 columns. The same phenomenon has been observed in 

 partially solidified beds of moist sand and clay. When the 

 rocks on one side slip along a crack so that the strata no 

 longer correspond (F, Fig. 42) it is termed a fault ; the 

 lower side is called the downthrow, the upper the upthrow. 

 Parallel lines 'of faults usually mark the borders of regions 

 where upheaval has taken place and the strata preserve a 

 low dip. When a fault shows at the surface no sudden rise 

 of level marks the upthrow side, as the action of erosion is 

 continually smoothing away such inequalities perhaps as 

 rapidly as they form. 



291. Temperature of the Earth's Crust. Whatever 

 be the nature of the surface rocks, the Sun's heat penetrates 

 them slightly and slowly. By observations in Britain with 

 thermometers fixed at various depths beneath the surface of 

 the land, it has been proved that the difference of day and 

 night temperatures vanishes at about 3 feet, and that the 

 greater and more regular difference between summer heat 

 and winter cold becomes less and less perceptible as the 

 distance increases, and dies away within 40 feet. The 

 average temperature shown by the rock thermometers on 



